diff --git a/rts/sm/GCUtils.c b/rts/sm/GCUtils.c
index 24f7b5e61e9e..31b2913a376c 100644
--- a/rts/sm/GCUtils.c
+++ b/rts/sm/GCUtils.c
@@ -81,6 +81,14 @@ freeChain_sync(bdescr *bd)
     RELEASE_SPIN_LOCK(&gc_alloc_block_sync);
 }
 
+void
+freeGroup_sync(bdescr *bd)
+{
+    ACQUIRE_SPIN_LOCK(&gc_alloc_block_sync);
+    freeGroup(bd);
+    RELEASE_SPIN_LOCK(&gc_alloc_block_sync);
+}
+
 /* -----------------------------------------------------------------------------
    Workspace utilities
    -------------------------------------------------------------------------- */
@@ -261,7 +269,7 @@ todo_block_full (uint32_t size, gen_workspace *ws)
                 // object.  However, if the object we're copying is
                 // larger than a block, then we might have an empty
                 // block here.
-                freeGroup(bd);
+                freeGroup_sync(bd);
             } else {
                 push_scanned_block(bd, ws);
             }
diff --git a/rts/sm/GCUtils.h b/rts/sm/GCUtils.h
index 3c174491cef0..8b6040769e0d 100644
--- a/rts/sm/GCUtils.h
+++ b/rts/sm/GCUtils.h
@@ -31,6 +31,7 @@ INLINE_HEADER bdescr *allocBlockOnNode_sync(uint32_t node)
 }
 
 void    freeChain_sync(bdescr *bd);
+void    freeGroup_sync(bdescr *bd);
 
 void    push_scanned_block   (bdescr *bd, gen_workspace *ws);
 StgPtr  todo_block_full      (uint32_t size, gen_workspace *ws);