diff --git a/test_conformance/extensions/cl_khr_command_buffer/command_buffer_printf.cpp b/test_conformance/extensions/cl_khr_command_buffer/command_buffer_printf.cpp
index eef3e3558..80fac2ada 100644
--- a/test_conformance/extensions/cl_khr_command_buffer/command_buffer_printf.cpp
+++ b/test_conformance/extensions/cl_khr_command_buffer/command_buffer_printf.cpp
@@ -426,7 +426,7 @@ struct CommandBufferPrintfTest : public BasicCommandBufferTest
                 std::max(min_pattern_length, rand() % max_pattern_length);
 
             std::vector<cl_char> pattern(pattern_length + 1, pattern_character);
-            pattern[pattern_length] = '\0';
+            pattern.back() = '\0';
             simul_passes[i] = { pattern,
                                 { cl_int(i * offset), cl_int(pattern_length) },
                                 std::vector<cl_char>(num_elements