diff --git a/inc/python b/inc/python
index 35bab6f..495150d 100644
--- a/inc/python
+++ b/inc/python
@@ -300,6 +300,13 @@
     local flags=$2
     local extras=$3
 
+    # Normalize the directory name to avoid
+    # "installation from path or url cannot be constrained to a version"
+    # error.
+    # REVISIT(yamamoto): Remove this when fixed in pip.
+    # https://github.com/pypa/pip/pull/3582
+    project_dir=$(cd $project_dir && pwd)
+
     if [ -n "$REQUIREMENTS_DIR" ]; then
         # Constrain this package to this project directory from here on out.
         local name