diff --git a/code/game/mecha/mech_fabricator.dm b/code/game/mecha/mech_fabricator.dm
index ec0fe428073..5a024fb7829 100644
--- a/code/game/mecha/mech_fabricator.dm
+++ b/code/game/mecha/mech_fabricator.dm
@@ -147,7 +147,7 @@
 
 	if(istype(I,/obj/item/stack/material))
 		var/obj/item/stack/material/S = I
-		if(!S.material in materials)
+		if(!(S.material in materials))
 			user << "<span class='warning'>The [src] doesn't accept [S.material]!</span>"
 			return
 
diff --git a/code/game/mecha/mech_prosthetics.dm b/code/game/mecha/mech_prosthetics.dm
index 65664d38783..173fd000129 100644
--- a/code/game/mecha/mech_prosthetics.dm
+++ b/code/game/mecha/mech_prosthetics.dm
@@ -174,7 +174,7 @@
 
 	if(istype(I,/obj/item/stack/material))
 		var/obj/item/stack/material/S = I
-		if(!S.material in materials)
+		if(!(S.material in materials))
 			user << "<span class='warning'>The [src] doesn't accept [S.material]!</span>"
 			return