diff --git a/src/AST.hs b/src/AST.hs
index 168a327a..636362a9 100644
--- a/src/AST.hs
+++ b/src/AST.hs
@@ -54,7 +54,8 @@ module AST (
   emptyInterface, emptyImplementation,
   getParams, getPrimParams, getDetism, getProcDef, getProcPrimProto,
   mkTempName, updateProcDef, updateProcDefM,
-  ModSpec, maybeModPrefix, ProcImpln(..), ProcDef(..), procInline, procCallCount,
+  ModSpec, validModuleName, maybeModPrefix, 
+  ProcImpln(..), ProcDef(..), procInline, procCallCount,
   transformModuleProcs,
   getProcGlobalFlows,
   primImpurity, flagsImpurity, flagsDetism,
@@ -1699,6 +1700,12 @@ instance Show TypeVarName where
 type ModSpec = [Ident]
 
 
+-- |Check that a module name component is valid (ie, does not contain invalid
+-- characters).  Currently only period (.) and hash (#) are considered invalid.
+validModuleName :: Ident -> Bool
+validModuleName = not . any (`elem` ['.','#'])
+
+
 -- |The uses one module makes of another; first the public imports,
 --  then the privates.  Each is either Nothing, meaning all exported
 --  names are imported, or Just a set of the specific names to import.
diff --git a/src/Normalise.hs b/src/Normalise.hs
index facc6627..b0827225 100644
--- a/src/Normalise.hs
+++ b/src/Normalise.hs
@@ -55,15 +55,18 @@ normalise items = do
 normaliseItem :: Item -> Compiler ()
 normaliseItem (TypeDecl vis (TypeProto name params) mods
               (TypeRepresentation rep) items pos) = do
+    validateModuleName "type" pos name
     let items' = RepresentationDecl params mods rep pos : items
     unless (List.null params)
       $ errmsg pos "types defined by representation cannot have type parameters"
     normaliseSubmodule name vis pos items'
 normaliseItem (TypeDecl vis (TypeProto name params) mods
               (TypeCtors ctorVis ctors) items pos) = do
+    validateModuleName "type" pos name
     let items' = ConstructorDecl ctorVis params mods ctors pos : items
     normaliseSubmodule name vis pos items'
-normaliseItem (ModuleDecl vis name items pos) =
+normaliseItem (ModuleDecl vis name items pos) = do
+    validateModuleName "module" pos name
     normaliseSubmodule name vis pos items
 normaliseItem (RepresentationDecl params mods rep pos) = do
     updateTypeModifiers mods
@@ -76,9 +79,12 @@ normaliseItem (ConstructorDecl vis params mods ctors pos) = do
         Public -> mapM_ (addConstructor Public . snd) ctors
         Private -> mapM_ (uncurry addConstructor) ctors
 normaliseItem (ImportMods vis modspecs pos) =
-    mapM_ (\spec -> addImport spec (importSpec Nothing vis)) modspecs
+    mapM_ (\spec -> validateModSpec pos spec >>
+        addImport spec (importSpec Nothing vis)
+    ) modspecs
 normaliseItem (ImportItems vis modspec imports pos) =
-    addImport modspec (importSpec (Just imports) vis)
+    validateModSpec pos modspec
+     >> addImport modspec (importSpec (Just imports) vis)
 normaliseItem (ImportForeign files _) =
     mapM_ addForeignImport files
 normaliseItem (ImportForeignLib files _) =
@@ -277,6 +283,18 @@ completeTypeSCC (CyclicSCC modTypeDefs) = do
     mapM_ (uncurry completeType) modTypeDefs
 
 
+-- |Check that the specified module name is valid, reporting and error if not.
+validateModuleName :: String -> OptPos -> Ident -> Compiler ()
+validateModuleName what pos name =
+    unless (validModuleName name)
+     $ errmsg pos $ "invalid character in " ++ what ++ " name `" ++ name ++ "`"
+
+
+-- |Check that the specified module name is valid, reporting and error if not.
+validateModSpec :: OptPos -> ModSpec -> Compiler ()
+validateModSpec pos = mapM_ (validateModuleName "module" pos) 
+
+
 -- | Information about a non-constant constructor
 data CtorInfo = CtorInfo {
            ctorInfoName   :: ProcName,        -- ^ this constructor's name
diff --git a/test-cases/final-dump/badmodname.exp b/test-cases/final-dump/badmodname.exp
new file mode 100644
index 00000000..8d19bf8b
--- /dev/null
+++ b/test-cases/final-dump/badmodname.exp
@@ -0,0 +1,3 @@
+Error detected during preliminary processing of module badmodname
+final-dump/badmodname.wybe:6:1: invalid character in module name `a.b`
+
diff --git a/test-cases/final-dump/badmodname.wybe b/test-cases/final-dump/badmodname.wybe
new file mode 100644
index 00000000..2d386906
--- /dev/null
+++ b/test-cases/final-dump/badmodname.wybe
@@ -0,0 +1,9 @@
+module a {
+    pub module b {
+        !print("in a.b")
+    }
+}
+module `a.b` {
+    !print("in `a.b`")
+}
+pass
\ No newline at end of file