diff --git a/mypy/main.py b/mypy/main.py
index a5511671c966..7edfff02e119 100644
--- a/mypy/main.py
+++ b/mypy/main.py
@@ -24,6 +24,10 @@
 PY_EXTENSIONS = tuple(PYTHON_EXTENSIONS)
 
 
+class InvalidPackageName(Exception):
+    """Exception indicating that a package name was invalid."""
+
+
 def main(script_path: str, args: List[str] = None) -> None:
     """Main entry point to the type checker.
 
@@ -457,9 +461,15 @@ def add_invertible_flag(flag: str,
         targets = []
         for f in special_opts.files:
             if f.endswith(PY_EXTENSIONS):
-                targets.append(BuildSource(f, crawl_up(f)[1], None))
+                try:
+                    targets.append(BuildSource(f, crawl_up(f)[1], None))
+                except InvalidPackageName as e:
+                    fail(str(e))
             elif os.path.isdir(f):
-                sub_targets = expand_dir(f)
+                try:
+                    sub_targets = expand_dir(f)
+                except InvalidPackageName as e:
+                    fail(str(e))
                 if not sub_targets:
                     fail("There are no .py[i] files in directory '{}'"
                          .format(f))
@@ -526,10 +536,14 @@ def crawl_up(arg: str) -> Tuple[str, str]:
         dir, base = os.path.split(dir)
         if not base:
             break
+        # Ensure that base is a valid python module name
+        if not base.isidentifier():
+            raise InvalidPackageName('{} is not a valid Python package name'.format(base))
         if mod == '__init__' or not mod:
             mod = base
         else:
             mod = base + '.' + mod
+
     return dir, mod
 
 
diff --git a/test-data/unit/cmdline.test b/test-data/unit/cmdline.test
index d0648844daaa..a9ba3c28950b 100644
--- a/test-data/unit/cmdline.test
+++ b/test-data/unit/cmdline.test
@@ -75,6 +75,14 @@ undef
 dir/subpkg/a.py:1: error: Name 'undef' is not defined
 dir/a.py:1: error: Name 'undef' is not defined
 
+[case testCmdlineInvalidPackageName]
+# cmd: mypy dir/sub.pkg/a.py
+[file dir/sub.pkg/__init__.py]
+[file dir/sub.pkg/a.py]
+undef
+[out]
+sub.pkg is not a valid Python package name
+
 [case testBadFileEncoding]
 # cmd: mypy a.py
 [file a.py]