diff --git a/src/helpers.js b/src/helpers.js
index 5102a83..56ad7ea 100644
--- a/src/helpers.js
+++ b/src/helpers.js
@@ -289,6 +289,11 @@ export function calculateExponent(num, e, exponent, isDecimal, precision) {
 		if (e < 0) {
 			e = 0;
 		}
+	} else if (e < 0) {
+		// A forced exponent below the auto sentinel (-1) has no meaning and
+		// would otherwise index the power-of-ten/two lookup tables out of
+		// bounds (producing NaN). Clamp to 0, mirroring the e > 8 clamp below.
+		e = 0;
 	}
 
 	if (e > 8) {
diff --git a/tests/unit/filesize.test.js b/tests/unit/filesize.test.js
index 035cb46..3e8ac6c 100644
--- a/tests/unit/filesize.test.js
+++ b/tests/unit/filesize.test.js
@@ -615,6 +615,23 @@ describe("filesize", () => {
 			const result = filesize(1024, { exponent: NaN, standard: "iec" });
 			assert.strictEqual(result, "1 KiB");
 		});
+
+		it("should clamp an out-of-range negative exponent to 0 instead of returning NaN", () => {
+			// Only -1 is the documented "auto" sentinel. Any other negative exponent
+			// indexed the power lookup tables out of bounds and produced "NaN undefined".
+			assert.strictEqual(filesize(1024, { exponent: -2 }), "1024 B");
+			assert.strictEqual(filesize(1024, { exponent: -100 }), "1024 B");
+		});
+
+		it("should clamp a negative exponent to 0 for object output", () => {
+			const result = filesize(1024, { exponent: -3, output: "object" });
+			assert.deepStrictEqual(result, { value: 1024, symbol: "B", exponent: 0, unit: "B" });
+		});
+
+		it("should clamp a negative exponent to 0 for array output", () => {
+			const result = filesize(1024, { exponent: -3, output: "array" });
+			assert.deepStrictEqual(result, [1024, "B"]);
+		});
 	});
 
 	describe("Error handling", () => {