Index: firmware/App/Modes/Dialysis.c
===================================================================
diff -u -re22cea17af706bcdaed705302094b096818147b0 -r88ee6f3dce69a3dadb012bb8a39c3fd167ba0afd
--- firmware/App/Modes/Dialysis.c	(.../Dialysis.c)	(revision e22cea17af706bcdaed705302094b096818147b0)
+++ firmware/App/Modes/Dialysis.c	(.../Dialysis.c)	(revision 88ee6f3dce69a3dadb012bb8a39c3fd167ba0afd)
@@ -79,7 +79,7 @@
 static F32 totalSalineVolumeDelivered;                  ///< Volume (mL) in total of saline delivered so far (cumulative for all boluses including current one).
 static F32 bolusSalineVolumeDelivered;                  ///< Volume (mL) of current bolus delivered so far.
 static F32 bolusSalineVolumeDelivered_Safety;           ///< Volume (mL) of current bolus delivered so far according to safety monitor.
-static U32 bolusSalineMotorCount;                       ///< Blood pump motor rev count during saline bolus to calculate saline bolus volume independently for safety.
+static S32 bolusSalineMotorCount;                       ///< Blood pump motor rev count during saline bolus to calculate saline bolus volume independently for safety.
 static U32 bolusSalineLastMotorCount;                   ///< Last blood pump count from last saline bolus volume update.
 static U32 bolusSalineLastVolumeTimeStamp;              ///< Time stamp for last saline volume update.
 
@@ -884,7 +884,7 @@
     bolusSalineLastVolumeTimeStamp = getMSTimerCount();
     bolusSalineVolumeDelivered += volSinceLastUpdateMl;
     totalSalineVolumeDelivered += volSinceLastUpdateMl;
-    bolusSalineMotorCount = u32DiffWithWrap( bolusSalineLastMotorCount, getBloodPumpMotorCount() ) / BP_HALL_EDGE_COUNTS_PER_REV;
+    bolusSalineMotorCount = u32BiDiffWithWrap( bolusSalineLastMotorCount, getBloodPumpMotorCount() ) / BP_HALL_EDGE_COUNTS_PER_REV;
     bolusSalineVolumeDelivered_Safety = ( (F32)bolusSalineMotorCount * VOLUME_PER_BP_MOTOR_REV_ML );  // TODO - include upstream pressure compensation to this calc (from PBA).
 
 #ifndef DISABLE_SALINE_BOLUS_CHECKS