diff --git a/src.ts/_tests/test-utils-units.ts b/src.ts/_tests/test-utils-units.ts
index d5089a45a1..ccd737ab85 100644
--- a/src.ts/_tests/test-utils-units.ts
+++ b/src.ts/_tests/test-utils-units.ts
@@ -2,7 +2,7 @@ import assert from "assert";
 
 import { loadTests } from "./utils.js";
 
-import { formatEther, formatUnits, parseEther, parseUnits } from "../index.js";
+import { FixedNumber, formatEther, formatUnits, parseEther, parseUnits } from "../index.js";
 
 import type { TestCaseUnit } from "./types.js";
 
@@ -70,4 +70,32 @@ describe("Tests bad unit conversion", function() {
             return error.message.startsWith("invalid unit");
         });
     });
+
+    it("correctly fails to convert negative decimal counts", function() {
+        assert.throws(() => {
+            formatUnits(123n, -1);
+        }, (error: any) => {
+            return error.message.startsWith("invalid FixedNumber decimals");
+        });
+
+        assert.throws(() => {
+            parseUnits("1.23", -1);
+        }, (error: any) => {
+            return error.message.startsWith("invalid FixedNumber decimals");
+        });
+    });
+
+    it("correctly fails to create FixedNumber values with negative decimals", function() {
+        assert.throws(() => {
+            FixedNumber.fromValue(1, -1);
+        }, (error: any) => {
+            return error.message.startsWith("invalid FixedNumber decimals");
+        });
+
+        assert.throws(() => {
+            FixedNumber.fromString("1.23", { decimals: -1, width: 128 });
+        }, (error: any) => {
+            return error.message.startsWith("invalid FixedNumber decimals");
+        });
+    });
 });
diff --git a/src.ts/utils/fixednumber.ts b/src.ts/utils/fixednumber.ts
index cc364af742..76f70f0e74 100644
--- a/src.ts/utils/fixednumber.ts
+++ b/src.ts/utils/fixednumber.ts
@@ -146,6 +146,7 @@ function getFormat(value?: FixedFormat): _FixedFormat {
     }
 
     assertArgument((width % 8) === 0, "invalid FixedNumber width (not byte aligned)", "format.width", width);
+    assertArgument(decimals >= 0, "invalid FixedNumber decimals (negative)", "format.decimals", decimals);
     assertArgument(decimals <= 80, "invalid FixedNumber decimals (too large)", "format.decimals", decimals);
 
     const name = (signed ? "": "u") + "fixed" + String(width) + "x" + String(decimals);
@@ -567,6 +568,7 @@ export class FixedNumber {
      */
     static fromValue(_value: BigNumberish, _decimals?: Numeric, _format?: FixedFormat): FixedNumber {
         const decimals = (_decimals == null) ? 0: getNumber(_decimals);
+        assertArgument(decimals >= 0, "invalid FixedNumber decimals (negative)", "decimals", decimals);
         const format = getFormat(_format);
 
         let value = getBigInt(_value, "value");