diff --git a/src/functions.js b/src/functions.js
index c90d2482..f2f0d706 100644
--- a/src/functions.js
+++ b/src/functions.js
@@ -825,6 +825,7 @@ const functions = (() => {
                         return subpicture.substring(0, ii);
                     }
                 }
+                return "";
             })();
             var suffix = (function () {
                 var ch;
@@ -834,6 +835,7 @@ const functions = (() => {
                         return subpicture.substring(ii + 1);
                     }
                 }
+                return "";
             })();
             var activePart = subpicture.substring(prefix.length, subpicture.length - suffix.length);
             var mantissaPart, exponentPart, integerPart, fractionalPart;
diff --git a/test/test-suite/groups/function-formatNumber/issue786.json b/test/test-suite/groups/function-formatNumber/issue786.json
new file mode 100644
index 00000000..2287d05e
--- /dev/null
+++ b/test/test-suite/groups/function-formatNumber/issue786.json
@@ -0,0 +1,26 @@
+[
+    {
+        "expr": "$formatNumber(42, '%%')",
+        "dataset": null,
+        "bindings": {},
+        "code": "D3086"
+    },
+    {
+        "expr": "$formatNumber(42, '‰‰')",
+        "dataset": null,
+        "bindings": {},
+        "code": "D3086"
+    },
+    {
+        "expr": "$formatNumber(42, '%‰')",
+        "dataset": null,
+        "bindings": {},
+        "code": "D3086"
+    },
+    {
+        "expr": "$formatNumber(42, '---')",
+        "dataset": null,
+        "bindings": {},
+        "code": "D3086"
+    }
+]
\ No newline at end of file