diff --git a/arch/inst/C/c.ebreak.yaml b/arch/inst/C/c.ebreak.yaml
index b39da5a89..959a64e18 100644
--- a/arch/inst/C/c.ebreak.yaml
+++ b/arch/inst/C/c.ebreak.yaml
@@ -21,7 +21,7 @@ definedBy:
   anyOf:
     - C
     - Zca
-assembly: " "
+assembly: ""
 encoding:
   match: "1001000000000010"
 access:
diff --git a/schemas/inst_schema.json b/schemas/inst_schema.json
index f1a8aa206..2400d72f2 100644
--- a/schemas/inst_schema.json
+++ b/schemas/inst_schema.json
@@ -401,7 +401,8 @@
         },
         "assembly": {
           "type": "string",
-          "description": "Assembly format of the instruction. Can use decode variables"
+          "description": "Assembly format of the instruction. Can use decode variables",
+          "pattern": "^$|^\\(?[a-zA-Z][a-zA-Z0-9_]*\\)?(?:, (imm\\(|\\()?[a-zA-Z][a-zA-Z0-9_]*\\)?)*$"
         },
         "data_independent_timing": {
           "type": "boolean",