diff --git a/libs/string/string.c b/libs/string/string.c
index f2fab4b..2feca4c 100644
--- a/libs/string/string.c
+++ b/libs/string/string.c
@@ -68,7 +68,7 @@ GroundValue stringSubstring(GroundScope* scope, List args) {
 
     if (start >= inputLen)
         ERROR("Start is outside string!", "OutOfBounds");
-    else if (end >= inputLen)
+    else if (end >= inputLen || end < 0)
         ERROR("End is outside string!", "OutOfBounds");
 
     char* buffer = malloc(inputLen + 1);