diff --git a/renderdoc/driver/shaders/spirv/spirv_debug_setup.cpp b/renderdoc/driver/shaders/spirv/spirv_debug_setup.cpp
index dcbcbf9b3..7b427ad9a 100644
--- a/renderdoc/driver/shaders/spirv/spirv_debug_setup.cpp
+++ b/renderdoc/driver/shaders/spirv/spirv_debug_setup.cpp
@@ -1794,7 +1794,8 @@ void Debugger::CalcActiveMask(rdcarray<bool> &activeMask)
     rdcarray<bool> inConverge;
     inConverge.resize(activeMask.size());
     for(size_t i = 0; i < workgroup.size(); i++)
-      inConverge[i] = (workgroup[i].callstack.back()->curBlock == convergeBlock);
+      inConverge[i] = (!workgroup[i].callstack.empty() &&
+                       workgroup[i].callstack.back()->curBlock == convergeBlock);
 
     // is any thread active, but not converged?
     bool anyActiveNotConverged = false;