diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index b3d2796d81..2e2f92fadf 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -2086,6 +2086,10 @@ void DerivationGoal::initChild()
                 if (mount("none", (chrootRootDir + "/dev/pts").c_str(), "devpts", 0, "newinstance,mode=0620") == -1)
                     throw SysError("mounting /dev/pts");
                 createSymlink("/dev/pts/ptmx", chrootRootDir + "/dev/ptmx");
+
+		/* Make sure /dev/pts/ptmx is world-writable.  With some
+		   Linux versions, it is created with permissions 0.  */
+		chmod_(chrootRootDir + "/dev/pts/ptmx", 0666);
             }
 
             /* Do the chroot().  Below we do a chdir() to the