diff --git a/site_ansto/motor_dmc2280.c b/site_ansto/motor_dmc2280.c
index da0b8087..592462de 100644
--- a/site_ansto/motor_dmc2280.c
+++ b/site_ansto/motor_dmc2280.c
@@ -774,11 +774,16 @@ static int checkMotion(void *pData) {
     return 1;
   }
   /* calculate observed and expected steps per count ratios */
-  ratio_obs = (steps - self->lastSteps) / (counts - self->lastCounts);
+  if (counts == self->lastCounts) /* prevent divide by zero */
+    ratio_obs = (steps - self->lastSteps);
+  else
+    ratio_obs = (steps - self->lastSteps) / (counts - self->lastCounts);
   ratio_exp = (float) self->stepsPerX / (float) self->cntsPerX;
   ratio_cmp = ratio_obs / ratio_exp;
-  /* less than half or more than double is trouble */
-  if (ratio_cmp > self->blockage_ratio || (1.0 / ratio_cmp) > self->blockage_ratio) {
+  /* wrong signs, less than half, or more than double is trouble */
+  if (ratio_cmp < 0.0 ||
+      ratio_cmp > self->blockage_ratio ||
+      (1.0 / ratio_cmp) > self->blockage_ratio) {
     char msg[132];
     snprintf(msg, sizeof(msg), "Motion check fail: obs=%f, exp=%f",
         ratio_obs, ratio_exp);