diff --git a/src/libstore/lock.cc b/src/libstore/lock.cc
index f1356fdca..fa718f55d 100644
--- a/src/libstore/lock.cc
+++ b/src/libstore/lock.cc
@@ -67,13 +67,26 @@ bool UserLock::findFreeUser() {
 #if __linux__
             /* Get the list of supplementary groups of this build user.  This
                is usually either empty or contains a group such as "kvm".  */
-            supplementaryGIDs.resize(10);
-            int ngroups = supplementaryGIDs.size();
-            int err = getgrouplist(pw->pw_name, pw->pw_gid,
-                supplementaryGIDs.data(), &ngroups);
-            if (err == -1)
-                throw Error("failed to get list of supplementary groups for '%1%'", pw->pw_name);
+            int ngroups = 32; // arbitrary initial guess
+            supplementaryGIDs.resize(ngroups);
 
+            int err = getgrouplist(pw->pw_name, pw->pw_gid, supplementaryGIDs.data(),
+                                 &ngroups);
+
+            // Our initial size of 32 wasn't sufficient, the correct size has
+            // been stored in ngroups, so we try again.
+            if (err == -1) {
+                supplementaryGIDs.resize(ngroups);
+                err = getgrouplist(pw->pw_name, pw->pw_gid, supplementaryGIDs.data(),
+                                   &ngroups);
+            }
+
+            // If it failed once more, then something must be broken.
+            if (err == -1)
+                throw Error("failed to get list of supplementary groups for '%1%'",
+                            pw->pw_name);
+
+            // Finally, trim back the GID list to its real size
             supplementaryGIDs.resize(ngroups);
 #endif