diff --git a/renderdoc/driver/gl/gl_overlay.cpp b/renderdoc/driver/gl/gl_overlay.cpp
index 8f5f4982d..6760a4822 100644
--- a/renderdoc/driver/gl/gl_overlay.cpp
+++ b/renderdoc/driver/gl/gl_overlay.cpp
@@ -373,26 +373,48 @@ ResourceId GLReplay::RenderOverlay(ResourceId texid, CompType typeHint, DebugOve
 
       const DrawcallDescription *draw = m_pDriver->GetDrawcall(eventId);
 
-      // readback the index buffer data
-      std::vector<byte> idxs;
-      uint32_t offset = draw->indexOffset * draw->indexByteWidth;
-      uint32_t length = 1;
-      drv.glGetNamedBufferParameterivEXT(idxbuf, eGL_BUFFER_SIZE, (GLint *)&length);
-
-      idxs.resize(length);
-      drv.glGetBufferSubData(eGL_ELEMENT_ARRAY_BUFFER, offset,
-                             RDCMIN(GLsizeiptr(length - offset),
-                                    GLsizeiptr(draw->numIndices) * GLsizeiptr(draw->indexByteWidth)),
-                             &idxs[0]);
-
-      // unbind the real index buffer
-      drv.glBindBuffer(eGL_ELEMENT_ARRAY_BUFFER, 0);
-
       std::vector<uint32_t> patchedIndices;
-      PatchLineStripIndexBuffer(
-          draw, draw->indexByteWidth == 1 ? (uint8_t *)idxs.data() : (uint8_t *)NULL,
-          draw->indexByteWidth == 2 ? (uint16_t *)idxs.data() : (uint16_t *)NULL,
-          draw->indexByteWidth == 4 ? (uint32_t *)idxs.data() : (uint32_t *)NULL, patchedIndices);
+
+      // readback the index buffer data
+      if(idxbuf)
+      {
+        std::vector<byte> idxs;
+        uint32_t offset = draw->indexOffset * draw->indexByteWidth;
+        uint32_t length = 1;
+        drv.glGetNamedBufferParameterivEXT(idxbuf, eGL_BUFFER_SIZE, (GLint *)&length);
+
+        idxs.resize(length);
+        drv.glGetBufferSubData(
+            eGL_ELEMENT_ARRAY_BUFFER, offset,
+            RDCMIN(GLsizeiptr(length - offset),
+                   GLsizeiptr(draw->numIndices) * GLsizeiptr(draw->indexByteWidth)),
+            &idxs[0]);
+
+        // unbind the real index buffer
+        drv.glBindBuffer(eGL_ELEMENT_ARRAY_BUFFER, 0);
+
+        uint32_t expectedSize = draw->numIndices * draw->indexByteWidth;
+
+        if(idxs.size() < expectedSize)
+        {
+          RDCERR("Index buffer is as large as expected");
+          idxs.resize(expectedSize);
+        }
+
+        PatchLineStripIndexBuffer(
+            draw, draw->indexByteWidth == 1 ? (uint8_t *)idxs.data() : (uint8_t *)NULL,
+            draw->indexByteWidth == 2 ? (uint16_t *)idxs.data() : (uint16_t *)NULL,
+            draw->indexByteWidth == 4 ? (uint32_t *)idxs.data() : (uint32_t *)NULL, patchedIndices);
+      }
+      else
+      {
+        // generate 'index' list
+        std::vector<uint32_t> idxs;
+        idxs.resize(draw->numIndices);
+        for(uint32_t i = 0; i < draw->numIndices; i++)
+          idxs[i] = i;
+        PatchLineStripIndexBuffer(draw, NULL, NULL, idxs.data(), patchedIndices);
+      }
 
       GLboolean primRestart = drv.glIsEnabled(eGL_PRIMITIVE_RESTART_FIXED_INDEX);
       drv.glEnable(eGL_PRIMITIVE_RESTART_FIXED_INDEX);