diff --git a/code/modules/DetectiveWork/footprints_and_rag.dm b/code/modules/DetectiveWork/footprints_and_rag.dm
index 1592f1c6f40..fd1f6a391d7 100644
--- a/code/modules/DetectiveWork/footprints_and_rag.dm
+++ b/code/modules/DetectiveWork/footprints_and_rag.dm
@@ -13,14 +13,14 @@
 
 
 
-/proc/blood_incompatible(donor,receiver)
-
-	var/donor_antigen = copytext(donor,1,lentext(donor))
-	var/receiver_antigen = copytext(receiver,1,lentext(receiver))
-	var/donor_rh = findtext("+",donor)
-	var/receiver_rh = findtext("+",receiver)
-
-	if(donor_rh && !receiver_rh) return 1
+proc/blood_incompatible(donor,receiver)
+	if(!donor || !receiver) return 0
+	var
+		donor_antigen = copytext(donor,1,lentext(donor))
+		receiver_antigen = copytext(receiver,1,lentext(receiver))
+		donor_rh = (findtext(donor,"+")>0)
+		receiver_rh = (findtext(receiver,"+")>0)
+	if(donor_rh!=receiver_rh) return 1
 	switch(receiver_antigen)
 		if("A")
 			if(donor_antigen != "A" && donor_antigen != "O") return 1