diff --git a/artiq/compiler/transforms/artiq_ir_generator.py b/artiq/compiler/transforms/artiq_ir_generator.py
index 0c753e984..330ff0ecf 100644
--- a/artiq/compiler/transforms/artiq_ir_generator.py
+++ b/artiq/compiler/transforms/artiq_ir_generator.py
@@ -821,7 +821,7 @@ class ARTIQIRGenerator(algorithm.Visitor):
                 timeout        = self.visit(context_expr_node.args[0])
                 timeout_ms     = self.append(ir.Arith(ast.Mult(loc=None), timeout,
                                                       ir.Constant(1000, builtins.TFloat())))
-                timeout_ms_int = self.append(ir.Coerce(timeout_ms, builtins.TInt32()))
+                timeout_ms_int = self.append(ir.Coerce(timeout_ms, builtins.TInt64()))
 
                 watchdog_id = self.append(ir.Builtin("watchdog_set", [timeout_ms_int],
                                                      builtins.TInt32()))
diff --git a/artiq/compiler/transforms/llvm_ir_generator.py b/artiq/compiler/transforms/llvm_ir_generator.py
index 1e5dfcde3..371d6ca29 100644
--- a/artiq/compiler/transforms/llvm_ir_generator.py
+++ b/artiq/compiler/transforms/llvm_ir_generator.py
@@ -356,7 +356,7 @@ class LLVMIRGenerator:
         elif name == "now":
             llty = lli64
         elif name == "watchdog_set":
-            llty = ll.FunctionType(lli32, [lli32])
+            llty = ll.FunctionType(lli32, [lli64])
         elif name == "watchdog_clear":
             llty = ll.FunctionType(llvoid, [lli32])
         else:
diff --git a/artiq/runtime.rs/libksupport/Cargo.lock b/artiq/runtime.rs/libksupport/Cargo.lock
index 042a9a750..da7d2b081 100644
--- a/artiq/runtime.rs/libksupport/Cargo.lock
+++ b/artiq/runtime.rs/libksupport/Cargo.lock
@@ -1,3 +1,4 @@
 [root]
 name = "ksupport"
 version = "0.0.0"
+
diff --git a/artiq/runtime.rs/libksupport/lib.rs b/artiq/runtime.rs/libksupport/lib.rs
index 4163482ab..5b12c360e 100644
--- a/artiq/runtime.rs/libksupport/lib.rs
+++ b/artiq/runtime.rs/libksupport/lib.rs
@@ -142,8 +142,11 @@ pub extern fn __artiq_terminate(exception: *const kernel_proto::Exception,
     loop {}
 }
 
-extern fn watchdog_set(ms: i32) -> usize {
-    // FIXME: fix ms
+extern fn watchdog_set(ms: i64) -> usize {
+    if ms < 0 {
+        artiq_raise!("ValueError", "cannot set a watchdog with a negative timeout")
+    }
+
     send(&WatchdogSetRequest { ms: ms as u64 });
     recv!(&WatchdogSetReply { id } => id)
 }
diff --git a/artiq/test/libartiq_support/artiq_time.c b/artiq/test/libartiq_support/artiq_time.c
index f60e46e47..07c81c9ac 100644
--- a/artiq/test/libartiq_support/artiq_time.c
+++ b/artiq/test/libartiq_support/artiq_time.c
@@ -3,7 +3,7 @@
 
 int64_t now = 0;
 
-int watchdog_set(int ms)
+int watchdog_set(long long ms)
 {
   printf("watchdog_set %d\n", ms);
   return ms;