diff --git a/src/syndicate.nim b/src/syndicate.nim
index 2658ad9..4af3041 100644
--- a/src/syndicate.nim
+++ b/src/syndicate.nim
@@ -196,6 +196,10 @@ method retract(e: ClosureEntity; turn: var Turn; h: Handle) =
 method message(e: ClosureEntity; turn: var Turn; v: Assertion) =
   if not e.messageImpl.isNil: e.messageImpl(turn, v)
 
+proc argumentCount(handler: NimNode): int =
+  handler.expectKind {nnkDo, nnkStmtList}
+  if handler.kind == nnkDo: result = pred handler[3].len
+
 proc wrapPublishHandler(handler: NimNode): NimNode =
   handler.expectKind {nnkDo, nnkStmtList}
   var innerProc = newNimNode(nnkProcDef)
@@ -268,18 +272,22 @@ proc wrapMessageHandler(handler: NimNode): NimNode =
 macro onPublish*(turn: Turn; ds: Ref; pattern: Pattern; handler: untyped) =
   ## Call `handler` when an assertion matching `pattern` is published at `ds`.
   let
+    argCount = argumentCount(handler)
     handlerProc = wrapPublishHandler(handler)
     handlerSym = handlerProc[0]
   result = quote do:
+    doAssert `pattern`.analyse.capturePaths.len == `argCount`, "mismatch between pattern capture and handler arguments"
     `handlerProc`
     discard observe(`turn`, `ds`, `pattern`, ClosureEntity(publishImpl: `handlerSym`))
 
 macro onMessage*(turn: Turn; ds: Ref; pattern: Pattern; handler: untyped) =
   ## Call `handler` when an message matching `pattern` is broadcasted at `ds`.
   let
+    argCount = argumentCount(handler)
     handlerProc = wrapMessageHandler(handler)
     handlerSym = handlerProc[0]
   result = quote do:
+    doAssert `pattern`.analyse.capturePaths.len == `argCount`, "mismatch between pattern capture and handler arguments"
     `handlerProc`
     discard observe(`turn`, `ds`, `pattern`, ClosureEntity(messageImpl: `handlerSym`))
 
@@ -341,17 +349,21 @@ macro during*(turn: var Turn; ds: Ref; pattern: Pattern; publishBody, retractBod
   ## - `bindings` - raw Preserves sequence that matched `pattern`
   ## - `duringHandle` - dataspace handle of the assertion that triggered `publishBody`
   let
+    argCount = argumentCount(publishBody)
     callbackProc = wrapDuringHandler(publishBody, retractBody)
     callbackSym = callbackProc[0]
   result = quote do:
+    doAssert `pattern`.analyse.capturePaths.len == `argCount`, "mismatch between pattern capture and handler arguments"
     `callbackProc`
     discard observe(`turn`, `ds`, `pattern`, during(`callbackSym`))
 
 macro during*(turn: var Turn; ds: Ref; pattern: Pattern; publishBody: untyped) =
   ## Variant of `during` without a retract body.
   let
+    argCount = argumentCount(publishBody)
     callbackProc = wrapDuringHandler(publishBody, nil)
     callbackSym = callbackProc[0]
   result = quote do:
+    doAssert `pattern`.analyse.capturePaths.len == `argCount`, "mismatch between pattern capture and handler arguments"
     `callbackProc`
     discard observe(`turn`, `ds`, `pattern`, during(`callbackSym`))