diff --git a/cmake/platforms/emscripten.cmake b/cmake/platforms/emscripten.cmake
index d74217f..b7e9c7d 100644
--- a/cmake/platforms/emscripten.cmake
+++ b/cmake/platforms/emscripten.cmake
@@ -49,6 +49,9 @@ set(emcc_export_list
   _rescale_puzzle
   # Callback for loading user preferences
   _prefs_load_callback
+  # Functions for allocating and freeing C memory
+  _malloc
+  _free
   # Main program, run at initialisation time
   _main)
 
diff --git a/emcclib.js b/emcclib.js
index bd4762d..d8ef57f 100644
--- a/emcclib.js
+++ b/emcclib.js
@@ -827,7 +827,13 @@ mergeInto(LibraryManager.library, {
             var prefsdata =
                 localStorage.getItem(location.pathname + " preferences");
             if (prefsdata !== undefined && prefsdata !== null) {
-                prefs_load_callback(me, prefsdata);
+                var lenbytes = lengthBytesUTF8(prefsdata) + 1;
+                var dest = _malloc(lenbytes);
+                if (dest != 0) {
+                    stringToUTF8(prefsdata, dest, lenbytes);
+                    prefs_load_callback(me, dest);
+                    _free(dest);
+                }
             }
         } catch (error) {
             // Log the error but otherwise pretend the settings were
diff --git a/emccpre.js b/emccpre.js
index a114ce1..842a392 100644
--- a/emccpre.js
+++ b/emccpre.js
@@ -693,7 +693,7 @@ function initPuzzle() {
                                    ['number','number']);
     timer_callback = Module.cwrap('timer_callback', 'void', ['number']);
     prefs_load_callback = Module.cwrap('prefs_load_callback', 'void',
-                                       ['number','string']);
+                                       ['number','number']);
 
     if (resizable_div !== null) {
         var resize_handle = document.getElementById("resizehandle");