diff --git a/8Knot/_bots.py b/8Knot/_bots.py
index eeeaaaa5..3b99a72c 100644
--- a/8Knot/_bots.py
+++ b/8Knot/_bots.py
@@ -15,14 +15,19 @@ def get_bots_list():
 
     try:
         dbm = AugurManager()
-        engine = dbm.get_engine()
+        dbm.get_engine()
     except KeyError:
-        # noack, data wasn't successfully set.
+        # defensive- currently unreachable, since app.py:38-44 constructs an
+        # AugurManager over the same credentials and sys.exit(1)s first.
+        # falling through from here left dbm unbound, so the caller would have
+        # seen a NameError rather than this.
         logging.error("BOT_DATA_QUERY - INCOMPLETE ENVIRONMENT")
+        raise
     except SQLAlchemyError:
+        # runs at import in the web process, so there's no Celery retry to
+        # fall back on- fail the boot rather than serve unfiltered bot data.
         logging.error("BOT_DATA_QUERY - COULDN'T CONNECT TO DB")
-        # allow retry via Celery rules.
-        raise SQLAlchemyError("DBConnect failed")
+        raise
 
     df = dbm.run_query(query_string)
     # reformat cntrb_id
diff --git a/8Knot/cache_manager/cache_facade.py b/8Knot/cache_manager/cache_facade.py
index 955b231d..120865f1 100644
--- a/8Knot/cache_manager/cache_facade.py
+++ b/8Knot/cache_manager/cache_facade.py
@@ -124,6 +124,14 @@ def get_uncached(func_name: str, repolist: list[int]) -> list[int]:  # or None
 
     Returns a list of repos that AREN'T resident in cache.
     """
+    if not repolist:
+        # nothing requested means nothing missing. guarded here rather than at
+        # each call site because psycopg2 renders an empty sequence as "in ()",
+        # which postgres rejects outright -- and every visualization callback
+        # polls this function on initial page load, when the repo-choices
+        # store still holds its default [].
+        return []
+
     with pg.connect(cache_cx_string) as cache_conn:
         with cache_conn.cursor() as cache_cur:
             composed_query = pg_sql.SQL(
@@ -181,17 +189,21 @@ def caching_wrapper(func_name: str, query: str, repolist: list[int], n_repolist_
             logging.warning(f"{func_name} COLLECTION - CACHING {len(uncached_repos)} NEW REPOS")
 
             # inject the repolist multiple times because the SQL uses it more
-            # than once and the wildcard %s are ordered.
-            uncached_repos: tuple[tuple] = tuple([tuple(uncached_repos) for _ in range(n_repolist_uses)])
+            # than once and the wildcard %s are ordered. kept under its own name
+            # so uncached_repos stays the list of repos this call is filling.
+            query_vars: tuple[tuple] = tuple([tuple(uncached_repos) for _ in range(n_repolist_uses)])
 
         # STEP 2: Query for those repos
         logging.warning(f"{func_name} COLLECTION - EXECUTING CACHING QUERY")
         cache_query_results(
             db_connection_string=db_cx_string,
             query=query,
-            vars=uncached_repos,
+            vars=query_vars,
             target_table=func_name,
-            bookkeeping_data=tuple({"cache_func": func_name, "repo_id": r} for r in repolist),
+            # only the repos actually cached here. recording the full repolist
+            # re-inserted a row for every repo that was already resident, and
+            # cache_bookkeeping has no unique constraint to absorb the repeats.
+            bookkeeping_data=tuple({"cache_func": func_name, "repo_id": r} for r in uncached_repos),
         )
     except Exception as e:
         logging.critical(f"{func_name}_POSTGRES ERROR: {e}")
@@ -224,7 +236,12 @@ def retrieve_from_cache(
                 """.format(
                     tablename=tablename
                 ),
-                (tuple(repolist),),
+                # psycopg2 renders an empty sequence as "()", which postgres
+                # rejects. NULL matches nothing, so an empty repolist returns
+                # zero rows while cursor.description still yields the table's
+                # columns -- callers preprocess on those columns before they
+                # test df.empty, so a 0x0 frame is not a safe substitute.
+                (tuple(repolist) or (None,),),
             )
 
             logging.warning(f"{tablename} - LOADING DATA FROM CACHE")
diff --git a/8Knot/db_manager/augur_manager.py b/8Knot/db_manager/augur_manager.py
index f0e11c44..440a182a 100644
--- a/8Knot/db_manager/augur_manager.py
+++ b/8Knot/db_manager/augur_manager.py
@@ -153,8 +153,12 @@ def run_query(self, query_string: str) -> pd.DataFrame:
         try:
             with self.engine.connect() as conn:
                 result_df = pd.read_sql(query, con=conn)
-        except:
-            raise Exception("DB Read Failure")
+        except Exception as e:
+            # a bare 'except' here also caught KeyboardInterrupt and SystemExit,
+            # and dropped the cause -- so a statement timeout, a bad column name
+            # and a dropped connection were indistinguishable in the logs.
+            logging.exception("AUGUR: DB read failed")
+            raise Exception("DB Read Failure") from e
 
         result_df = result_df.reset_index()
         result_df.drop("index", axis=1, inplace=True)
diff --git a/8Knot/pages/codebase/visualizations/cntrb_file_heatmap.py b/8Knot/pages/codebase/visualizations/cntrb_file_heatmap.py
index 606899c9..384348f0 100644
--- a/8Knot/pages/codebase/visualizations/cntrb_file_heatmap.py
+++ b/8Knot/pages/codebase/visualizations/cntrb_file_heatmap.py
@@ -110,6 +110,9 @@ def repo_dropdown(repo_ids):
     background=True,
 )
 def directory_dropdown(repo_id):
+    if repo_id is None:
+        return [], None
+
     # Convert to int since Mantine dropdown returns strings
     repo_id = int(repo_id)
 
@@ -190,6 +193,9 @@ def cntrb_file_heatmap_graph(searchbar_repos, repo_id, directory, bot_switch):
     start = time.perf_counter()
     logging.warning(f"{VIZ_ID}- START")
 
+    if repo_id is None:
+        return nodata_graph
+
     # Convert to int since Mantine dropdown returns strings
     repo_id = int(repo_id)
 
diff --git a/8Knot/pages/codebase/visualizations/contribution_file_heatmap.py b/8Knot/pages/codebase/visualizations/contribution_file_heatmap.py
index aa700281..c14d2df5 100644
--- a/8Knot/pages/codebase/visualizations/contribution_file_heatmap.py
+++ b/8Knot/pages/codebase/visualizations/contribution_file_heatmap.py
@@ -125,6 +125,9 @@ def repo_dropdown(repo_ids):
     background=True,
 )
 def directory_dropdown(repo_id):
+    if repo_id is None:
+        return [], None
+
     # Convert to int since Mantine dropdown returns strings
     repo_id = int(repo_id)
 
@@ -200,6 +203,9 @@ def cntrb_file_heatmap_graph(repo_id, directory, graph_view):
     start = time.perf_counter()
     logging.warning(f"{VIZ_ID}- START")
 
+    if repo_id is None:
+        return nodata_graph
+
     # Convert to int since Mantine dropdown returns strings
     repo_id = int(repo_id)
 
diff --git a/8Knot/pages/codebase/visualizations/reviewer_file_heatmap.py b/8Knot/pages/codebase/visualizations/reviewer_file_heatmap.py
index 956fbff6..5f6cc5b0 100644
--- a/8Knot/pages/codebase/visualizations/reviewer_file_heatmap.py
+++ b/8Knot/pages/codebase/visualizations/reviewer_file_heatmap.py
@@ -110,6 +110,9 @@ def repo_dropdown(repo_ids):
     background=True,
 )
 def directory_dropdown(repo_id):
+    if repo_id is None:
+        return [], None
+
     # Convert to int since Mantine dropdown returns strings
     repo_id = int(repo_id)
 
@@ -190,6 +193,9 @@ def reviewer_file_heatmap_graph(searchbar_repos, repo_id, directory, bot_switch)
     start = time.perf_counter()
     logging.warning(f"{VIZ_ID}- START")
 
+    if repo_id is None:
+        return nodata_graph
+
     # Convert to int since Mantine dropdown returns strings
     repo_id = int(repo_id)
 
diff --git a/8Knot/pages/repo_overview/repo_overview.py b/8Knot/pages/repo_overview/repo_overview.py
index 12ea99fc..298dbd7b 100644
--- a/8Knot/pages/repo_overview/repo_overview.py
+++ b/8Knot/pages/repo_overview/repo_overview.py
@@ -78,4 +78,4 @@ def repo_dropdown(repo_ids):
     for repo_id in repo_ids:
         entry = {"value": str(repo_id), "label": augur.repo_id_to_git(int(repo_id))}
         data_array.append(entry)
-    return data_array, str(repo_ids[0])
+    return data_array, str(repo_ids[0]) if repo_ids else None
diff --git a/8Knot/pages/repo_overview/visualizations/ossf_scorecard.py b/8Knot/pages/repo_overview/visualizations/ossf_scorecard.py
index 30a5ec07..5a7baa4a 100644
--- a/8Knot/pages/repo_overview/visualizations/ossf_scorecard.py
+++ b/8Knot/pages/repo_overview/visualizations/ossf_scorecard.py
@@ -99,8 +99,10 @@ def toggle_popover(n, is_open):
 )
 def ossf_scorecard(repo: str):
 
-    if repo is not None:
-        repo = int(repo)
+    if repo is None:
+        return dbc.Table.from_dataframe(pd.DataFrame(), striped=True, bordered=True, hover=True), dbc.Label("No data")
+
+    repo = int(repo)
 
     # wait for data to asynchronously download and become available.
     while not_cached := cf.get_uncached(func_name=osq.__name__, repolist=[repo]):
diff --git a/8Knot/pages/repo_overview/visualizations/repo_general_info.py b/8Knot/pages/repo_overview/visualizations/repo_general_info.py
index a0924d56..de0a5f5b 100644
--- a/8Knot/pages/repo_overview/visualizations/repo_general_info.py
+++ b/8Knot/pages/repo_overview/visualizations/repo_general_info.py
@@ -80,8 +80,10 @@ def toggle_popover(n, is_open):
 )
 def repo_general_info(repo):
 
-    if repo is not None:
-        repo = int(repo)
+    if repo is None:
+        return dbc.Table.from_dataframe(pd.DataFrame(), striped=True, bordered=True, hover=True), dbc.Label("No data")
+
+    repo = int(repo)
 
     logging.warning(f"{VIZ_ID} - START")
     start = time.perf_counter()
diff --git a/8Knot/pages/utils/job_utils.py b/8Knot/pages/utils/job_utils.py
index 7219b9a0..409ce633 100644
--- a/8Knot/pages/utils/job_utils.py
+++ b/8Knot/pages/utils/job_utils.py
@@ -51,9 +51,15 @@ def get_default_repo_with_data(repo_ids, cache_tablename):
         cache_tablename: Name of the cache table to query
 
     Returns:
-        str: The first repo_id (as string) that has cached data, or repo_ids[0] as fallback
+        str | None: The first repo_id with cached data, repo_ids[0] as fallback, or None when no repos are selected
     """
 
+    if not repo_ids:
+        # no selection to default to. this runs in the web process on initial
+        # page load, when the repo-choices store still holds its default [],
+        # and every return below indexes repo_ids[0].
+        return None
+
     df = cf.retrieve_from_cache(tablename=cache_tablename, repolist=repo_ids)
 
     if df.empty: