diff --git a/src/pocketpy.cpp b/src/pocketpy.cpp
index 27455846..37603f5b 100644
--- a/src/pocketpy.cpp
+++ b/src/pocketpy.cpp
@@ -627,6 +627,7 @@ void init_builtins(VM* _vm) {
         const Str& self = _CAST(Str&, args[0]);
         const Str& value = CAST(Str&, args[1]);
         int start = CAST(int, args[2]);
+        if (start < 0) vm->ValueError("argument 'start' can't be negative");
         int index = self.index(value, start);
         if(index < 0) vm->ValueError("substring not found");
         return VAR(index);
@@ -636,6 +637,7 @@ void init_builtins(VM* _vm) {
         const Str& self = _CAST(Str&, args[0]);
         const Str& value = CAST(Str&, args[1]);
         int start = CAST(int, args[2]);
+		if (start < 0) vm->ValueError("argument 'start' can't be negative");
         return VAR(self.index(value, start));
     });
 
diff --git a/tests/04_str.py b/tests/04_str.py
index 3a588229..c9c78eac 100644
--- a/tests/04_str.py
+++ b/tests/04_str.py
@@ -250,6 +250,17 @@ try:
 except ValueError:
     pass
 
+try:
+    a.index('1', -1)
+    exit(1)
+except ValueError:
+    pass
+
 assert a.find('1') == 0
 assert a.find('1', 1) == -1
 
+try:
+    a.find('1', -1)
+    exit(1)
+except ValueError:
+    pass