diff --git a/core/rospy/cmake/rospy.cmake b/core/rospy/cmake/rospy.cmake
index db38ca2c..617f6456 100644
--- a/core/rospy/cmake/rospy.cmake
+++ b/core/rospy/cmake/rospy.cmake
@@ -15,10 +15,16 @@ macro(genmsg_py)
     set(_output_py ${PROJECT_SOURCE_DIR}/src/${PROJECT_NAME}/msg/_${_msg})
     string(REPLACE ".msg" ".py" _output_py ${_output_py})
   
-    # Add the rule to build the .h the .msg
+    # Add the rule to build the .h the .msg.
+    #
+    # Note the dependency on ${_autogen}.  This is done to force serial
+    # execution of the message-generation steps, because genmsg_py is not
+    # parallel-safe.  When that's fixed
+    # (https://code.ros.org/trac/ros/ticket/1827), this dependency  can be
+    # removed.
     add_custom_command(OUTPUT ${_output_py} 
                        COMMAND ${genmsg_py_exe} ${_input}
-                       DEPENDS ${_input} ${genmsg_py_exe} ${gendeps_exe} ${${PROJECT_NAME}_${_msg}_GENDEPS} ${ROS_MANIFEST_LIST})
+                       DEPENDS ${_input} ${genmsg_py_exe} ${gendeps_exe} ${${PROJECT_NAME}_${_msg}_GENDEPS} ${ROS_MANIFEST_LIST} ${_autogen})
     list(APPEND _autogen ${_output_py})
   endforeach(_msg)
   # Create a target that depends on the union of all the autogenerated
@@ -48,9 +54,15 @@ macro(gensrv_py)
     string(REPLACE ".srv" ".py" _output_py ${_output_py})
   
     # Add the rule to build the .h from the .srv
+    #
+    # Note the dependency on ${_autogen}.  This is done to force serial
+    # execution of the service-generation steps, because genmsg_py is not
+    # parallel-safe.  When that's fixed
+    # (https://code.ros.org/trac/ros/ticket/1827), this dependency  can be
+    # removed.
     add_custom_command(OUTPUT ${_output_py} 
                        COMMAND ${gensrv_py_exe} ${_input}
-                       DEPENDS ${_input} ${gensrv_py_exe} ${gendeps_exe} ${${PROJECT_NAME}_${_srv}_GENDEPS} ${ROS_MANIFEST_LIST})
+                       DEPENDS ${_input} ${gensrv_py_exe} ${gendeps_exe} ${${PROJECT_NAME}_${_srv}_GENDEPS} ${ROS_MANIFEST_LIST} ${_autogen})
     list(APPEND _autogen ${_output_py})
   endforeach(_srv)
   # Create a target that depends on the union of all the autogenerated