diff --git a/src/vm/wren_utils.c b/src/vm/wren_utils.c
index b5c31888..1b366561 100644
--- a/src/vm/wren_utils.c
+++ b/src/vm/wren_utils.c
@@ -161,18 +161,16 @@ int wrenUtf8Decode(const uint8_t* bytes, uint32_t length)
   return value;
 }
 
-int wrenUtf8DecodeNumBytes(const char* string, uint32_t index)
+int wrenUtf8DecodeNumBytes(uint8_t byte)
 {
-  char first = string[index];
-  
   // If the byte starts with 10xxxxx, it's the middle of a UTF-8 sequence, so
   // don't count it at all.
-  if ((first & 0xc0) == 0x80) return 0;
+  if ((byte & 0xc0) == 0x80) return 0;
   
   // The first byte's high bits tell us how many bytes are in the UTF-8
   // sequence.
-  if ((first & 0xf8) == 0xf0) return 4;
-  if ((first & 0xf0) == 0xe0) return 3;
-  if ((first & 0xe0) == 0xc0) return 2;
+  if ((byte & 0xf8) == 0xf0) return 4;
+  if ((byte & 0xf0) == 0xe0) return 3;
+  if ((byte & 0xe0) == 0xc0) return 2;
   return 1;
 }
diff --git a/src/vm/wren_utils.h b/src/vm/wren_utils.h
index 469162c1..a1f2e2fd 100644
--- a/src/vm/wren_utils.h
+++ b/src/vm/wren_utils.h
@@ -114,11 +114,10 @@ int wrenUtf8Encode(int value, uint8_t* bytes);
 // Returns -1 if the bytes are not a valid UTF-8 sequence.
 int wrenUtf8Decode(const uint8_t* bytes, uint32_t length);
 
-// Returns the number of bytes in the UTF-8 sequence starting at [index] in
-// [string].
+// Returns the number of bytes in the UTF-8 sequence starting with [byte].
 //
 // If the character at that index is not the beginning of a UTF-8 sequence,
 // returns 0.
-int wrenUtf8DecodeNumBytes(const char* string, uint32_t index);
+int wrenUtf8DecodeNumBytes(uint8_t byte);
 
 #endif
diff --git a/src/vm/wren_value.c b/src/vm/wren_value.c
index dec3c080..3eec04d9 100644
--- a/src/vm/wren_value.c
+++ b/src/vm/wren_value.c
@@ -638,7 +638,7 @@ Value wrenNewStringFromRange(WrenVM* vm, ObjString* source, int start,
   int length = 0;
   for (uint32_t i = 0; i < count; i++)
   {
-    length += wrenUtf8EncodeNumBytes(from[start + i * step]);
+    length += wrenUtf8DecodeNumBytes(from[start + i * step]);
   }
   
   ObjString* result = allocateString(vm, length);
@@ -655,7 +655,7 @@ Value wrenNewStringFromRange(WrenVM* vm, ObjString* source, int start,
       to += wrenUtf8Encode(codePoint, to);
     }
   }
-
+  
   hashString(result);
   return OBJ_VAL(result);
 }
@@ -772,7 +772,7 @@ Value wrenStringCodePointAt(WrenVM* vm, ObjString* string, uint32_t index)
 {
   ASSERT(index < string->length, "Index out of bounds.");
   
-  int numBytes = wrenUtf8DecodeNumBytes(string->value, index);
+  int numBytes = wrenUtf8DecodeNumBytes(string->value[index]);
   return wrenNewString(vm, string->value + index, numBytes);
 }