diff --git a/include/graphblas/omp/config.hpp b/include/graphblas/omp/config.hpp
index 5c5cdb076..106528d68 100644
--- a/include/graphblas/omp/config.hpp
+++ b/include/graphblas/omp/config.hpp
@@ -79,6 +79,15 @@ namespace grb {
 					return ret;
 				}
 
+				/**
+				 * @returns The maximum number of threads reported by OpenMP.
+				 *
+				 * This function can be called from any context.
+				 */
+				static size_t maxThreads() {
+					return omp_get_max_threads();
+				}
+
 				/**
 				 * @returns The number of threads in the current OpenMP parallel section.
 				 *
diff --git a/include/graphblas/reference/coordinates.hpp b/include/graphblas/reference/coordinates.hpp
index 59ea0326f..60253723b 100644
--- a/include/graphblas/reference/coordinates.hpp
+++ b/include/graphblas/reference/coordinates.hpp
@@ -97,6 +97,13 @@ namespace grb {
 				/** Per-thread capacity for parallel stack updates. */
 				size_t _buf;
 
+				/** Number of threads for which these coordinates have been initialised. */
+#ifdef _H_GRB_REFERENCE_OMP_COORDINATES
+				const size_t _threads = config::OMP::threads();
+#else
+				const size_t _threads = 1;
+#endif
+
 				/**
 				 * Increments the number of nonzeroes in the current thread-local stack.
 				 *
@@ -294,6 +301,10 @@ namespace grb {
 					// blocks are not managed by this class)
 				}
 
+				size_t requiredThreadsForUpdate() const noexcept {
+					return _threads;
+				}
+
 				/**
 				 * @returns An empty thread-local stack for new nonzeroes.
 				 */
diff --git a/include/graphblas/reference/vector.hpp b/include/graphblas/reference/vector.hpp
index f0db908b2..ac6284330 100644
--- a/include/graphblas/reference/vector.hpp
+++ b/include/graphblas/reference/vector.hpp
@@ -1274,6 +1274,14 @@ namespace grb {
 
 			template< typename D, typename C >
 			inline C & getCoordinates( Vector< D, reference, C > &x ) noexcept {
+#if defined(_H_GRB_REFERENCE_OMP_VECTOR) && !defined(NDEBUG)
+				if( x._coordinates.requiredThreadsForUpdate() != config::OMP::maxThreads() ) {
+					#pragma omp critical
+						std::cerr << "   " << x._coordinates.requiredThreadsForUpdate()
+							<< " != " << config::OMP::maxThreads() << "\n";
+				}
+				assert( x._coordinates.requiredThreadsForUpdate() == config::OMP::maxThreads() );
+#endif
 				return x._coordinates;
 			}