diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index 572634765..e0c7324c7 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -3678,7 +3678,8 @@ void DerivationGoal::registerOutputs()
                 /* The output path should be a regular file without execute permission. */
                 if (!S_ISREG(st.st_mode) || (st.st_mode & S_IXUSR) != 0)
                     throw BuildError(
-                        format("output path '%1%' should be a non-executable regular file") % path);
+                        format("output path '%1%' should be a non-executable regular file "
+                               "since recursive hashing is not enabled (outputHashMode=flat).") % path);
             }
 
             /* Check the hash. In hash mode, move the path produced by