diff --git a/doit/dependency.py b/doit/dependency.py
index 84973144..85092ac8 100644
--- a/doit/dependency.py
+++ b/doit/dependency.py
@@ -36,6 +36,9 @@ def get_file_md5(path):
     @param path: (string) file path
     @return: (string) md5
     """
+    if os.path.isdir(path):
+        raise ValueError("Cannot compute MD5 sum of directory. You must "
+                         "specify a file as a dependency instead.")
     with open(path, 'rb') as file_data:
         md5 = hashlib.md5()
         block_size = 128 * md5.block_size
diff --git a/tests/test_dependency.py b/tests/test_dependency.py
index 38d71f42..93c6457c 100644
--- a/tests/test_dependency.py
+++ b/tests/test_dependency.py
@@ -34,6 +34,9 @@ def test_md5():
     expected_crlf = "cf7b48b2fec3b581b135f7c9a1f7ae04"
     assert get_file_md5(filePath) in {expected_lf, expected_crlf}
 
+    with pytest.raises(ValueError):
+        get_file_md5(os.path.dirname(filePath))
+
 
 def test_sqlite_import():
     """