diff --git a/libs/pika/async_cuda/tests/unit/cublas_matmul.cpp b/libs/pika/async_cuda/tests/unit/cublas_matmul.cpp
index 1091e123e..aada709c9 100644
--- a/libs/pika/async_cuda/tests/unit/cublas_matmul.cpp
+++ b/libs/pika/async_cuda/tests/unit/cublas_matmul.cpp
@@ -287,7 +287,7 @@ int pika_main(pika::program_options::variables_map& vm)
     sizeMult = (std::min)(sizeMult, std::size_t(100));
     sizeMult = (std::max)(sizeMult, std::size_t(1));
     //
-    int block_size = 32;
+    std::size_t block_size = 32;
 
     sMatrixSize matrix_size;
     matrix_size.uiWA = 2 * block_size * sizeMult;