diff --git a/src/renderer_gl.cpp b/src/renderer_gl.cpp
index 8f91f649..b8d3c206 100644
--- a/src/renderer_gl.cpp
+++ b/src/renderer_gl.cpp
@@ -3410,6 +3410,10 @@ namespace bgfx { namespace gl
 				m_used[used++] = ii;
 			}
 		}
+		BX_CHECK(used < BX_COUNTOF(m_used), "Out of bounds %d > array size %d."
+				, used
+				, BX_COUNTOF(m_used)
+				);
 		m_used[used] = Attrib::Count;
 
 		used = 0;
@@ -3422,6 +3426,10 @@ namespace bgfx { namespace gl
 				m_instanceData[used++] = loc;
 			}
 		}
+		BX_CHECK(used < BX_COUNTOF(m_instanceData), "Out of bounds %d > array size %d."
+				, used
+				, BX_COUNTOF(m_instanceData)
+				);
 		m_instanceData[used] = 0xffff;
 	}
 
diff --git a/src/renderer_gl.h b/src/renderer_gl.h
index 8c77fd25..7bd186cf 100644
--- a/src/renderer_gl.h
+++ b/src/renderer_gl.h
@@ -1061,7 +1061,7 @@ namespace bgfx { namespace gl
 
 		uint8_t m_used[Attrib::Count+1]; // dense
 		GLint m_attributes[Attrib::Count]; // sparse
-		GLint m_instanceData[BGFX_CONFIG_MAX_INSTANCE_DATA_COUNT];
+		GLint m_instanceData[BGFX_CONFIG_MAX_INSTANCE_DATA_COUNT+1];
 
  		GLint m_sampler[BGFX_CONFIG_MAX_TEXTURE_SAMPLERS];
  		uint8_t m_numSamplers;