diff --git a/ChangeLog b/ChangeLog
index 4ccc86792..13faf3249 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -49,6 +49,8 @@
             ESP32C3: don't kick connected BLE devices off if reset() is called
             ESP32C3: disable warnings about "BT_BTM: BTM_GetSecurityFlags false"
             Handle semicolons between field declarations in classes
+            Ensure that pretokenised Strings are loaded as Flat Strings when executed from a String in RAM
+            E.nativeCall also checks to see if getting data as a flat string failed (could have caused segfault)
             
      2v24 : Bangle.js2: Add 'Bangle.touchRd()', 'Bangle.touchWr()'
             Bangle.js2: After Bangle.showTestScreen, put Bangle.js into a hard off state (not soft off)
diff --git a/src/jsvar.c b/src/jsvar.c
index d1b8ac458..22c995ba3 100644
--- a/src/jsvar.c
+++ b/src/jsvar.c
@@ -1632,21 +1632,9 @@ JsVar *jsvAsStringAndUnLock(JsVar *var) {
 JsVar *jsvAsFlatString(JsVar *var) {
   if (jsvIsFlatString(var)) return jsvLockAgain(var);
   JsVar *str = jsvAsString(var);
-  size_t len = jsvGetStringLength(str);
-  JsVar *flat = jsvNewFlatStringOfLength((unsigned int)len);
-  if (flat) {
-    JsvStringIterator src;
-    JsvStringIterator dst;
-    jsvStringIteratorNew(&src, str, 0);
-    jsvStringIteratorNew(&dst, flat, 0);
-    while (len--) {
-      jsvStringIteratorSetCharAndNext(&dst, jsvStringIteratorGetCharAndNext(&src));
-    }
-    jsvStringIteratorFree(&src);
-    jsvStringIteratorFree(&dst);
-  }
+  JsVar *fs = jsvNewFlatStringFromStringVar(str, 0, JSVAPPENDSTRINGVAR_MAXLENGTH);
   jsvUnLock(str);
-  return flat;
+  return fs;
 }
 
 /** Given a JsVar meant to be an index to an array, convert it to
@@ -1943,7 +1931,29 @@ void jsvAppendStringVar(JsVar *var, const JsVar *str, size_t stridx, size_t maxL
   jsvStringIteratorFree(&dst);
 }
 
-/** Create a new String from a substring in RAM. It is always writable. jsvNewFromStringVar can reference a non-writable string.
+/** Create a new flat string from the given var with the given index and length */
+JsVar *jsvNewFlatStringFromStringVar(JsVar *var, size_t stridx, size_t maxLength) {
+  assert(jsvIsString(var));
+  size_t len = jsvGetStringLength(var);
+  if (stridx>len) len=0;
+  else len -= stridx;
+  if (len > maxLength) len = maxLength;
+  JsVar *flat = jsvNewFlatStringOfLength((unsigned int)len);
+  if (flat) {
+    JsvStringIterator src;
+    JsvStringIterator dst;
+    jsvStringIteratorNew(&src, var, stridx);
+    jsvStringIteratorNew(&dst, flat, 0);
+    while (len--) {
+      jsvStringIteratorSetCharAndNext(&dst, jsvStringIteratorGetCharAndNext(&src));
+    }
+    jsvStringIteratorFree(&src);
+    jsvStringIteratorFree(&dst);
+  }
+  return flat;
+}
+
+/** Create a new (non-flat) String from a substring in RAM. It is always writable and appendable. jsvNewFromStringVar can reference a non-writable string.
 The Argument must be a string. stridx = start char or str, maxLength = max number of characters (can be JSVAPPENDSTRINGVAR_MAXLENGTH) */
 JsVar *jsvNewWritableStringFromStringVar(const JsVar *str, size_t stridx, size_t maxLength) {
   JsVar *var = jsvNewFromEmptyString();
@@ -1958,6 +1968,7 @@ JsVar *jsvNewWritableStringFromStringVar(const JsVar *str, size_t stridx, size_t
 /** Create a new variable from a substring. If a Native or Flash String, the memory area will be referenced (so the new string may not be writable)
 The Argument must be a string. stridx = start char or str, maxLength = max number of characters (can be JSVAPPENDSTRINGVAR_MAXLENGTH) */
 JsVar *jsvNewFromStringVar(const JsVar *str, size_t stridx, size_t maxLength) {
+  assert(jsvIsString(str));
   if (jsvIsNativeString(str) || jsvIsFlashString(str)) {
     // if it's a flash string, just change the pointer (but we must check length)
     size_t l = jsvGetStringLength(str);
@@ -1968,6 +1979,18 @@ JsVar *jsvNewFromStringVar(const JsVar *str, size_t stridx, size_t maxLength) {
     res->varData.nativeStr.len = (JsVarDataNativeStrLength)maxLength;
     return res;
   }
+  if (jsvIsFlatString(str)) {
+    // work out how long we really want it...
+    size_t length = jsvGetCharactersInVar(str);
+    if (stridx >= length) length = 0;
+    else length -= stridx;
+    if (length > maxLength) length = maxLength;
+    // if it's long enough to make sense, create a flat string instead
+    if (length > JSV_FLAT_STRING_BREAK_EVEN) {
+      JsVar *var = jsvNewFlatStringFromStringVar(str, stridx, length);
+      if (var) return var;
+    }
+  }
   return jsvNewWritableStringFromStringVar(str, stridx, maxLength);
 }
 
diff --git a/src/jsvar.h b/src/jsvar.h
index 86d3a635f..6d802393f 100644
--- a/src/jsvar.h
+++ b/src/jsvar.h
@@ -318,7 +318,9 @@ JsVar *jsvNewUTF8String(JsVar* dataString); ///< Create a new unicode string usi
 JsVar *jsvNewUTF8StringAndUnLock(JsVar* dataString); ///< Create a new unicode string using the given data string for backing
 #endif
 static ALWAYS_INLINE JsVar *jsvNewNull() { return jsvNewWithFlags(JSV_NULL); } ;///< Create a new null variable
-/** Create a new String from a substring in RAM. It is always writable. jsvNewFromStringVar can reference a non-writable string.
+/** Create a new flat string from the given var with the given index and length */
+JsVar *jsvNewFlatStringFromStringVar(JsVar *var, size_t stridx, size_t maxLength);
+/** Create a new (non-flat) String from a substring in RAM. It is always writable and appendable. jsvNewFromStringVar can reference a non-writable string.
 The Argument must be a string. stridx = start char or str, maxLength = max number of characters (can be JSVAPPENDSTRINGVAR_MAXLENGTH) */
 JsVar *jsvNewWritableStringFromStringVar(const JsVar *str, size_t stridx, size_t maxLength);
 /** Create a new variable from a substring. If a Native or Flash String, the memory area will be referenced (so the new string may not be writable)
diff --git a/src/jswrap_espruino.c b/src/jswrap_espruino.c
index d4f1f0a8e..bace3f856 100644
--- a/src/jswrap_espruino.c
+++ b/src/jswrap_espruino.c
@@ -281,6 +281,10 @@ JsVar *jswrap_espruino_nativeCall(JsVarInt addr, JsVar *signature, JsVar *data)
   JsVar *fn = jsvNewNativeFunction((void *)(size_t)addr, (unsigned short)argTypes);
   if (data) {
     JsVar *flat = jsvAsFlatString(data);
+    if (!flat) { // can't get a flat string!
+      jsvUnLock(fn);
+      return 0;
+    }
     jsvAddNamedChildAndUnLock(fn, flat, JSPARSE_FUNCTION_CODE_NAME);
   }
   return fn;