diff --git a/Cargo.lock b/Cargo.lock
index f19cf3c..5f9b877 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -114,6 +114,12 @@ dependencies = [
  "winapi",
 ]
 
+[[package]]
+name = "once_cell"
+version = "1.3.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b1c601810575c99596d4afc46f78a678c80105117c379eb3650cf99b8a21ce5b"
+
 [[package]]
 name = "piet"
 version = "0.0.12"
@@ -149,6 +155,7 @@ name = "piet-gpu-hal"
 version = "0.1.0"
 dependencies = [
  "ash",
+ "once_cell",
 ]
 
 [[package]]
diff --git a/piet-gpu-hal/examples/collatz.rs b/piet-gpu-hal/examples/collatz.rs
index 7195891..a4777b4 100644
--- a/piet-gpu-hal/examples/collatz.rs
+++ b/piet-gpu-hal/examples/collatz.rs
@@ -17,6 +17,7 @@ fn main() {
         let query_pool = device.create_query_pool(2).unwrap();
         let mut cmd_buf = device.create_cmd_buf().unwrap();
         cmd_buf.begin();
+        cmd_buf.reset_query_pool(&query_pool);
         cmd_buf.write_timestamp(&query_pool, 0);
         cmd_buf.dispatch(&pipeline, &descriptor_set, (256, 1, 1));
         cmd_buf.write_timestamp(&query_pool, 1);
diff --git a/piet-gpu-hal/src/lib.rs b/piet-gpu-hal/src/lib.rs
index 77170c0..d215490 100644
--- a/piet-gpu-hal/src/lib.rs
+++ b/piet-gpu-hal/src/lib.rs
@@ -80,6 +80,13 @@ pub trait CmdBuf<D: Device> {
 
     unsafe fn copy_buffer(&self, src: &D::Buffer, dst: &D::Buffer);
 
+    /// Reset the query pool.
+    ///
+    /// The query pool must be reset before each use, to avoid validation errors.
+    /// This is annoying, and we could tweak the API to make it implicit, doing
+    /// the reset before the first timestamp write.
+    unsafe fn reset_query_pool(&mut self, pool: &D::QueryPool);
+
     unsafe fn write_timestamp(&mut self, pool: &D::QueryPool, query: u32);
 }
 
diff --git a/piet-gpu-hal/src/vulkan.rs b/piet-gpu-hal/src/vulkan.rs
index 587f0a6..f5d8c19 100644
--- a/piet-gpu-hal/src/vulkan.rs
+++ b/piet-gpu-hal/src/vulkan.rs
@@ -573,6 +573,16 @@ impl crate::CmdBuf<VkDevice> for CmdBuf {
         );
     }
 
+    unsafe fn reset_query_pool(&mut self, pool: &QueryPool) {
+        let device = &self.device.device;
+        device.cmd_reset_query_pool(
+            self.cmd_buf,
+            pool.pool,
+            0,
+            pool.n_queries,
+        );
+    }
+
     unsafe fn write_timestamp(&mut self, pool: &QueryPool, query: u32) {
         let device = &self.device.device;
         device.cmd_write_timestamp(
diff --git a/piet-gpu/src/main.rs b/piet-gpu/src/main.rs
index 949ccc2..9f4f25f 100644
--- a/piet-gpu/src/main.rs
+++ b/piet-gpu/src/main.rs
@@ -181,6 +181,7 @@ fn main() {
         cmd_buf.clear_buffer(&tilegroup_buf);
         cmd_buf.clear_buffer(&ptcl_buf);
         cmd_buf.memory_barrier();
+        cmd_buf.reset_query_pool(&query_pool);
         cmd_buf.write_timestamp(&query_pool, 0);
         cmd_buf.dispatch(
             &k1_pipeline,