diff --git a/src/builtin/string.rs b/src/builtin/string.rs
index 9ffc169..637bc6a 100644
--- a/src/builtin/string.rs
+++ b/src/builtin/string.rs
@@ -186,10 +186,16 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
             } else if index == Number::from(0) {
                 string.insert_str(0, &substr);
             } else {
-                string.insert_str(
-                    len - index.abs().to_integer().to_usize().unwrap().min(len),
-                    &substr,
-                );
+                let idx = index.abs().to_integer().to_usize().unwrap();
+                if idx > len {
+                    string.insert_str(0, &substr)
+                } else {
+                    string.insert_str(
+                        len - idx + 1,
+                        &substr,
+                    );
+
+                }
             }
 
             Ok(Value::Ident(string, quotes))
diff --git a/tests/strings.rs b/tests/strings.rs
index 8ac1181..c263ede 100644
--- a/tests/strings.rs
+++ b/tests/strings.rs
@@ -150,9 +150,19 @@ test!(
     "a {\n  color: Xabcd;\n}\n"
 );
 test!(
-    str_insert_negative_idx,
-    "a {\n  color: str-insert(abcd, \"X\", -2);\n}\n",
-    "a {\n  color: abXcd;\n}\n"
+    str_insert_negative_1,
+    "a {\n  color: str-insert(abc, \"X\", -1);\n}\n",
+    "a {\n  color: abcX;\n}\n"
+);
+test!(
+    str_insert_negative_2,
+    "a {\n  color: str-insert(abc, \"X\", -2);\n}\n",
+    "a {\n  color: abXc;\n}\n"
+);
+test!(
+    str_insert_negative_3,
+    "a {\n  color: str-insert(abc, \"X\", -3);\n}\n",
+    "a {\n  color: aXbc;\n}\n"
 );
 error!(
     float_idx,