diff --git a/changes/ticket33290 b/changes/ticket33290
new file mode 100644
index 0000000000000000000000000000000000000000..882764020ebe14603f79f7b7a707fe0b595fb63f
--- /dev/null
+++ b/changes/ticket33290
@@ -0,0 +1,4 @@
+  o Minor features (diagnostic):
+    - Improve assertions and add some memory-poisoning code to try to track
+      down possible causes of a rare crash (32564) in the EWMA code.
+      Closes ticket 33290.
diff --git a/src/core/or/circuitmux.c b/src/core/or/circuitmux.c
index b2628bec3f2bbc72ac4fc7f7ba35b2cbaf6f1605..72f6ba662bc7b87093ad6a349d9e14e4b2d349f9 100644
--- a/src/core/or/circuitmux.c
+++ b/src/core/or/circuitmux.c
@@ -79,6 +79,8 @@
 #include "core/or/destroy_cell_queue_st.h"
 #include "core/or/or_circuit_st.h"
 
+#include "lib/crypt_ops/crypto_util.h"
+
 /*
  * Private typedefs for circuitmux.c
  */
@@ -973,7 +975,10 @@ circuitmux_detach_circuit,(circuitmux_t *cmux, circuit_t *circ))
     /* Now remove it from the map */
     HT_REMOVE(chanid_circid_muxinfo_map, cmux->chanid_circid_map, hashent);
 
-    /* Free the hash entry */
+    /* Wipe and free the hash entry */
+    // This isn't sensitive, but we want to be sure to know if we're accessing
+    // this accidentally.
+    memwipe(hashent, 0xef, sizeof(hashent));
     tor_free(hashent);
   }
 }
@@ -1334,4 +1339,3 @@ circuitmux_compare_muxes, (circuitmux_t *cmux_1, circuitmux_t *cmux_2))
     return 0;
   }
 }
-
diff --git a/src/core/or/circuitmux_ewma.c b/src/core/or/circuitmux_ewma.c
index 3f83c3fd5a7034f26e3c068f11e8f92fb46ec61d..606b755e28ffa8df074e127e311947a8e82c0ff7 100644
--- a/src/core/or/circuitmux_ewma.c
+++ b/src/core/or/circuitmux_ewma.c
@@ -147,7 +147,9 @@ TO_EWMA_POL_DATA(circuitmux_policy_data_t *pol)
 {
   if (!pol) return NULL;
   else {
-    tor_assert(pol->magic == EWMA_POL_DATA_MAGIC);
+    tor_assertf(pol->magic == EWMA_POL_DATA_MAGIC,
+                "Mismatch: %"PRIu32" != %"PRIu32,
+                pol->magic, EWMA_POL_DATA_MAGIC);
     return DOWNCAST(ewma_policy_data_t, pol);
   }
 }
@@ -162,7 +164,9 @@ TO_EWMA_POL_CIRC_DATA(circuitmux_policy_circ_data_t *pol)
 {
   if (!pol) return NULL;
   else {
-    tor_assert(pol->magic == EWMA_POL_CIRC_DATA_MAGIC);
+    tor_assertf(pol->magic == EWMA_POL_CIRC_DATA_MAGIC,
+                "Mismatch: %"PRIu32" != %"PRIu32,
+                pol->magic, EWMA_POL_CIRC_DATA_MAGIC);
     return DOWNCAST(ewma_policy_circ_data_t, pol);
   }
 }
@@ -295,6 +299,7 @@ ewma_free_cmux_data(circuitmux_t *cmux,
   pol = TO_EWMA_POL_DATA(pol_data);
 
   smartlist_free(pol->active_circuit_pqueue);
+  pol->base_.magic = 0xDEAD901C;
   tor_free(pol);
 }
 
@@ -361,7 +366,7 @@ ewma_free_circ_data(circuitmux_t *cmux,
   if (!pol_circ_data) return;
 
   cdata = TO_EWMA_POL_CIRC_DATA(pol_circ_data);
-
+  cdata->base_.magic = 0xDEADC14C;
   tor_free(cdata);
 }