Index: sys/dev/hwpmc/hwpmc_mod.c
===================================================================
--- sys/dev/hwpmc/hwpmc_mod.c
+++ sys/dev/hwpmc/hwpmc_mod.c
@@ -4199,6 +4199,7 @@
 	struct pmc_samplebuffer *psb;
 #ifdef	INVARIANTS
 	int ncallchains;
+	int nfree;
 #endif
 
 	psb = pmc_pcpu[cpu]->pc_sb[ring];
@@ -4210,6 +4211,7 @@
 
 #ifdef	INVARIANTS
 	ncallchains = 0;
+	nfree = 0;
 #endif
 
 	/*
@@ -4221,6 +4223,10 @@
 	ps = psb->ps_read;
 	ps_end = psb->ps_write;
 	do {
+#ifdef	INVARIANTS
+		if (ps->ps_pmc->pm_state != PMC_STATE_RUNNING)
+			nfree++;
+#endif
 		if (ps->ps_nsamples != PMC_SAMPLE_INUSE)
 			goto next;
 		if (ps->ps_td != td)
@@ -4256,7 +4262,7 @@
 			ps = psb->ps_samples;
 	} while (ps != ps_end);
 
-	KASSERT(ncallchains > 0,
+	KASSERT(ncallchains > 0 || nfree > 0,
 	    ("[pmc,%d] cpu %d didn't find a sample to collect", __LINE__,
 		cpu));