diff --git a/src/mame/video/n64.c b/src/mame/video/n64.c
index 6edb92cb2c9..8a8373ffc59 100644
--- a/src/mame/video/n64.c
+++ b/src/mame/video/n64.c
@@ -1983,6 +1983,9 @@ void n64_rdp::DrawTriangle(bool shade, bool texture, bool zbuffer, bool rect)
 	int dzdx_dz = (dzdx >> 16) & 0xffff;
 
 	extent_t Spans[2048];
+#ifdef MAME_DEBUG
+	memset(Spans, 0xcc, sizeof(Spans));
+#endif
 
 	SpanBase.m_span_drdy = drdy;
 	SpanBase.m_span_dgdy = dgdy;
diff --git a/src/mame/video/rdpspn16.c b/src/mame/video/rdpspn16.c
index d1ac11eb437..8a1f23faac4 100644
--- a/src/mame/video/rdpspn16.c
+++ b/src/mame/video/rdpspn16.c
@@ -152,6 +152,11 @@ void n64_rdp::SpanDraw1Cycle(INT32 scanline, const extent_t &extent, const rdp_p
 
 	INT32 tile1 = tilenum;
 
+#ifdef PTR64
+	assert(extent.userdata != (const void *)0xcccccccccccccccc);
+#else
+	assert(extent.userdata != (const void *)0xcccccccc);
+#endif
 	rdp_span_aux *userdata = (rdp_span_aux*)extent.userdata;
 
 	INT32 m_clamp_s_diff[8];
@@ -325,6 +330,11 @@ void n64_rdp::SpanDraw2Cycle(INT32 scanline, const extent_t &extent, const rdp_p
 	INT32 news = 0;
 	INT32 newt = 0;
 
+#ifdef PTR64
+	assert(extent.userdata != (const void *)0xcccccccccccccccc);
+#else
+	assert(extent.userdata != (const void *)0xcccccccc);
+#endif
 	rdp_span_aux *userdata = (rdp_span_aux*)extent.userdata;
 
 	INT32 m_clamp_s_diff[8];
@@ -367,6 +377,9 @@ void n64_rdp::SpanDraw2Cycle(INT32 scanline, const extent_t &extent, const rdp_p
 		dzinc = 0;
 	}
 
+	if (object.MiscState.FBSize < 2 || object.MiscState.FBSize > 4)
+		fatalerror("unsupported FBSize %d\n", object.MiscState.FBSize);
+
 	int blend_index = (object.OtherModes.alpha_cvg_select ? 2 : 0) | ((object.OtherModes.rgb_dither_sel < 3) ? 1 : 0);
 	int read_index = ((object.MiscState.FBSize - 2) << 1) | object.OtherModes.image_read_en;
 	int write_index = ((object.MiscState.FBSize - 2) << 3) | (object.OtherModes.cvg_dest << 1);