diff --git a/src/builtin/string.rs b/src/builtin/string.rs
index 179dd19..9ffc169 100644
--- a/src/builtin/string.rs
+++ b/src/builtin/string.rs
@@ -161,7 +161,13 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
             };
 
             let index = match arg!(args, 2, "index") {
-                Value::Dimension(n, _) => n.to_integer().to_usize().unwrap(),
+                Value::Dimension(n, Unit::None) if n.is_decimal() => {
+                    return Err(format!("$index: {} is not an int.", n).into())
+                }
+                Value::Dimension(n, Unit::None) => n,
+                v @ Value::Dimension(..) => {
+                    return Err(format!("$index: Expected {} to have no units.", v).into())
+                }
                 v => return Err(format!("$index: {} is not a number.", v).into()),
             };
 
@@ -170,7 +176,21 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
                 QuoteKind::None => QuoteKind::None,
             };
 
-            string.insert_str(index - 1, &substr);
+            let len = string.len();
+
+            if index > Number::from(0) {
+                string.insert_str(
+                    index.to_integer().to_usize().unwrap().min(len + 1) - 1,
+                    &substr,
+                );
+            } 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,
+                );
+            }
 
             Ok(Value::Ident(string, quotes))
         }),
diff --git a/tests/strings.rs b/tests/strings.rs
index f4e7567..8ac1181 100644
--- a/tests/strings.rs
+++ b/tests/strings.rs
@@ -144,3 +144,32 @@ test!(
     "a {\n  color: str-insert(abcd, \"\", 4);\n}\n",
     "a {\n  color: abcd;\n}\n"
 );
+test!(
+    str_insert_idx_0,
+    "a {\n  color: str-insert(abcd, \"X\", 0);\n}\n",
+    "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"
+);
+error!(
+    float_idx,
+    "a {\n  color: str-insert(abcd, \"X\", .5);\n}\n", "Error: $index: 0.5 is not an int."
+);
+error!(
+    idx_with_units,
+    "a {\n  color: str-insert(abcd, \"X\", 5px);\n}\n",
+    "Error: $index: Expected 5px to have no units."
+);
+test!(
+    idx_larger_than_string,
+    "a {\n  color: str-insert(abcd, \"X\", 20);\n}\n",
+    "a {\n  color: abcdX;\n}\n"
+);
+test!(
+    idx_larger_than_string_negative,
+    "a {\n  color: str-insert(abcd, \"X\", -20);\n}\n",
+    "a {\n  color: Xabcd;\n}\n"
+);