diff --git a/builtins.go b/builtins.go
index 0e6812d..eaee396 100644
--- a/builtins.go
+++ b/builtins.go
@@ -1179,7 +1179,7 @@ func builtinSubstr(i *interpreter, inputStr, inputFrom, inputLen value) (value,
 	}
 
 	fromInt := int(fromV.value)
-	strStr := strV.getGoString()
+	strStr := strV.getRunes()
 
 	endIndex := fromInt + lenInt
 
@@ -1190,9 +1190,7 @@ func builtinSubstr(i *interpreter, inputStr, inputFrom, inputLen value) (value,
 	if fromInt > len(strStr) {
 		return makeValueString(""), nil
 	}
-
-	runes := []rune(strStr)
-	return makeValueString(string(runes[fromInt:endIndex])), nil
+	return makeValueString(string(strStr[fromInt:endIndex])), nil
 }
 
 func builtinSplitLimit(i *interpreter, strv, cv, maxSplitsV value) (value, error) {
diff --git a/testdata/builtin_substr_multibyte.golden b/testdata/builtin_substr_multibyte.golden
new file mode 100644
index 0000000..19ea4ad
--- /dev/null
+++ b/testdata/builtin_substr_multibyte.golden
@@ -0,0 +1 @@
+"🤩"
diff --git a/testdata/builtin_substr_multibyte.jsonnet b/testdata/builtin_substr_multibyte.jsonnet
new file mode 100644
index 0000000..ea7b51f
--- /dev/null
+++ b/testdata/builtin_substr_multibyte.jsonnet
@@ -0,0 +1 @@
+std.substr("🤩", 0, 5)
\ No newline at end of file
diff --git a/testdata/builtin_substr_multibyte.linter.golden b/testdata/builtin_substr_multibyte.linter.golden
new file mode 100644
index 0000000..e69de29