diff --git a/vasl_templates/webapp/vasl_mod.py b/vasl_templates/webapp/vasl_mod.py
index ccd2034..3387a3a 100644
--- a/vasl_templates/webapp/vasl_mod.py
+++ b/vasl_templates/webapp/vasl_mod.py
@@ -25,6 +25,12 @@ _warnings = [] # nb: for the test suite
 
 # ---------------------------------------------------------------------
 
+class CantFindDataDirException( Exception ):
+    """Raised if the VASL data directory is missing."""
+    def __init__( self, vasl_version ):
+        super().__init__()
+        self.vasl_version = vasl_version
+
 def set_vasl_mod( vmod_fname, msg_store ):
     """Install a new global VaslMod object."""
     globvars.vasl_mod = None
@@ -36,6 +42,13 @@ def set_vasl_mod( vmod_fname, msg_store ):
         extns = _load_vasl_extns( extns_dir, msg_store )
         try:
             vasl_mod = VaslMod( vmod_fname, DATA_DIR, extns )
+        except CantFindDataDirException as ex:
+            msg = "Can't find the data directory for VASL {}." \
+                  " Are you running an older version of this program?".format( ex.vasl_version )
+            _logger.error( "%s", msg )
+            if msg_store:
+                msg_store.error( msg )
+            return
         except Exception as ex: #pylint: disable=broad-except
             msg = "Can't load the VASL module: {}".format( ex )
             _logger.error( "%s", msg )
@@ -265,11 +278,20 @@ class VaslMod:
             aliases = json.load( fp )
         self.vasl_version = aliases.get( self.vasl_real_version, self.vasl_real_version )
 
+        # locate the VASL data files
+        # NOTE: We do this explicitly because people were running older versions of vasl-templates, configured
+        # with a version of VASL that it didn't know about. An exception was thrown when we tried to open
+        # the data files below, which was caught and reported correctly, but the error message was a little opaque,
+        # so we check for this case explicitly, and try to show something a little more comprehensible :-/
+        dname = os.path.join( data_dir, "vasl-"+self.vasl_version )
+        if not os.path.isdir( dname ):
+            raise CantFindDataDirException( self.vasl_version )
+
         # load our overrides
-        fname = os.path.join( data_dir, "vasl-"+self.vasl_version, "vasl-overrides.json" )
+        fname = os.path.join( data_dir, dname, "vasl-overrides.json" )
         with open( fname, "r", encoding="utf-8" ) as fp:
             vasl_overrides = json.load( fp )
-        fname = os.path.join( data_dir, "vasl-"+self.vasl_version, "expected-multiple-images.json" )
+        fname = os.path.join( data_dir, dname, "expected-multiple-images.json" )
         with open( fname, "r", encoding="utf-8" ) as fp:
             expected_multiple_images = json.load( fp )
 
@@ -277,7 +299,7 @@ class VaslMod:
         target_gpids = get_vo_gpids( self )
 
         # parse the VASL module and any extensions
-        fname = os.path.join( data_dir, "vasl-"+self.vasl_version, "piece-info.json" )
+        fname = os.path.join( data_dir, dname, "piece-info.json" )
         with open( fname, "r", encoding="utf-8" ) as fp:
             piece_info = json.load( fp )
         for i,files in enumerate( self._files ):