diff --git a/assert.go b/assert.go
index 6560ba0..5011797 100644
--- a/assert.go
+++ b/assert.go
@@ -237,7 +237,11 @@ func formatMsgAndArgs(dflt string, msgAndArgs ...any) string {
 	if len(msgAndArgs) == 0 {
 		return dflt
 	}
-	return fmt.Sprintf(msgAndArgs[0].(string), msgAndArgs[1:]...)
+	format, ok := msgAndArgs[0].(string)
+	if !ok {
+		panic("message argument to assert function must be a fmt string")
+	}
+	return fmt.Sprintf(format, msgAndArgs[1:]...)
 }
 
 func needlePosition(haystack, needle string) (quotedHaystack, quotedNeedle, positions string) {
diff --git a/assert_test.go b/assert_test.go
index 00bdfa3..20bf37a 100644
--- a/assert_test.go
+++ b/assert_test.go
@@ -139,6 +139,12 @@ func TestIsError(t *testing.T) {
 	})
 }
 
+func TestInvalidFormatMsg(t *testing.T) {
+	Panics(t, func() {
+		NotZero(t, Data{}, 123)
+	})
+}
+
 func TestNotIsError(t *testing.T) {
 	assertFail(t, "SameError", func(t testing.TB) {
 		NotIsError(t, fmt.Errorf("os error: %w", os.ErrClosed), os.ErrClosed)