diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index 6258bcb33..73e09cb10 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -3681,7 +3681,8 @@ 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