diff --git a/lib/bcdice/game_system/Cthulhu.rb b/lib/bcdice/game_system/Cthulhu.rb
index 7dc06e7e8..cefa947f0 100644
--- a/lib/bcdice/game_system/Cthulhu.rb
+++ b/lib/bcdice/game_system/Cthulhu.rb
@@ -193,12 +193,12 @@ def compare(total, target, broken_number = 0)
       end
 
       def getRegistResult(command)
-        m = /^RES(B)?([-\d]+)$/i.match(command)
+        m = /^RESB?(-?\d+)$/i.match(command)
         unless m
           return nil
         end
 
-        value = m[2].to_i
+        value = m[1].to_i
         target = value * 5 + 50
 
         if target < 5
diff --git a/test/data/Cthulhu.toml b/test/data/Cthulhu.toml
index 4fe236a7e..47d0dd762 100644
--- a/test/data/Cthulhu.toml
+++ b/test/data/Cthulhu.toml
@@ -162,6 +162,12 @@ rands = [
   { sides = 100, value = 47 },
 ]
 
+[[ test ]]
+game_system = "Cthulhu"
+input = "RES18-11"
+output = ""
+rands = []
+
 
 ##### 組み合わせロール CBR #####
 
@@ -820,6 +826,12 @@ rands = [
 ]
 
 
+[[ test ]]
+game_system = "Cthulhu"
+input = "RESB18-11"
+output = ""
+rands = []
+
 ##### 戦闘用組み合わせロール CBRB #####
 
 [[ test ]]