Index: trunk/psModules/src/imcombine/pmSubtraction.c
===================================================================
--- trunk/psModules/src/imcombine/pmSubtraction.c	(revision 20465)
+++ trunk/psModules/src/imcombine/pmSubtraction.c	(revision 20466)
@@ -1229,4 +1229,9 @@
 #endif
 
+    // Need to turn off threads at the psLib level --- otherwise, we end up with threads on top of threads,
+    // and everything is executing psThreadPoolWait, waiting for some other mythical thread to complete the
+    // thread's work.
+    psImageConvolveSetThreads(false);
+
     for (int j = yMin; j < yMax; j += fullSize) {
         int ySubMax = PS_MIN(j + fullSize, yMax); // Range for subregion of interest