diff --git a/Source/Core/VideoCommon/IndexGenerator.cpp b/Source/Core/VideoCommon/IndexGenerator.cpp
index 5d86561ffd..03d0815a13 100644
--- a/Source/Core/VideoCommon/IndexGenerator.cpp
+++ b/Source/Core/VideoCommon/IndexGenerator.cpp
@@ -328,12 +328,29 @@ void IndexGenerator::AddExternalIndices(const u16* indices, u32 num_indices, u32
 
 u32 IndexGenerator::GetRemainingIndices(OpcodeDecoder::Primitive primitive) const
 {
-  u32 max_index = USHRT_MAX;
+  u32 max_index = UINT16_MAX;
 
   if (g_Config.UseVSForLinePointExpand() && primitive >= OpcodeDecoder::Primitive::GX_DRAW_LINES)
     max_index >>= 2;
 
-  // -1 is reserved for primitive restart
+  // Although we reserve UINT16_MAX for primitive restart, we aren't allowed to use that as an
+  // actual index. But, the maximum number of vertices a game can send is UINT16_MAX, so up to
+  // 0xffff indices will be used by the game. These indices would be 0x0000 through 0xfffe,
+  // and since m_base_index gets incremented for each index used, after that m_base_index
+  // would be 0xffff and no indices remain. If a game uses 0xfffe vertices, assuming m_base_index
+  // started at 0 it would end at 0xfffe and one more index could be used. So, we do not need to
+  // subtract 1 from max_index to correctly reserve one index for primitive restart.
+  //
+  // Pocoyo Racing uses a draw command with 0xffff vertices, which previously caused issues; see
+  // https://bugs.dolphin-emu.org/issues/13136 for details.
 
-  return max_index - m_base_index - 1;
+  if (m_base_index > max_index) [[unlikely]]
+  {
+    PanicAlertFmt("GetRemainingIndices would overflow; we've already written too many indices? "
+                  "base index {} > max index {}",
+                  m_base_index, max_index);
+    return 0;
+  }
+
+  return max_index - m_base_index;
 }
diff --git a/Source/Core/VideoCommon/VertexManagerBase.cpp b/Source/Core/VideoCommon/VertexManagerBase.cpp
index 88d7c47fba..85e2b48556 100644
--- a/Source/Core/VideoCommon/VertexManagerBase.cpp
+++ b/Source/Core/VideoCommon/VertexManagerBase.cpp
@@ -140,26 +140,11 @@ DataReader VertexManagerBase::PrepareForAdditionalData(OpcodeDecoder::Primitive
   }
 
   // Check for size in buffer, if the buffer gets full, call Flush()
-  if (!m_is_flushed &&
-      (count > m_index_generator.GetRemainingIndices(primitive) ||
-       count > GetRemainingIndices(primitive) || needed_vertex_bytes > GetRemainingSize()))
+  if (!m_is_flushed && (count > m_index_generator.GetRemainingIndices(primitive) ||
+                        count > GetRemainingIndices(primitive) ||
+                        needed_vertex_bytes > GetRemainingSize())) [[unlikely]]
   {
     Flush();
-
-    if (count > m_index_generator.GetRemainingIndices(primitive))
-    {
-      ERROR_LOG_FMT(VIDEO, "Too little remaining index values. Use 32-bit or reset them on flush.");
-    }
-    if (count > GetRemainingIndices(primitive))
-    {
-      ERROR_LOG_FMT(VIDEO, "VertexManager: Buffer not large enough for all indices! "
-                           "Increase MAXIBUFFERSIZE or we need primitive breaking after all.");
-    }
-    if (needed_vertex_bytes > GetRemainingSize())
-    {
-      ERROR_LOG_FMT(VIDEO, "VertexManager: Buffer not large enough for all vertices! "
-                           "Increase MAXVBUFFERSIZE or we need primitive breaking after all.");
-    }
   }
 
   m_cull_all = cullall;
@@ -182,6 +167,23 @@ DataReader VertexManagerBase::PrepareForAdditionalData(OpcodeDecoder::Primitive
     m_is_flushed = false;
   }
 
+  // Now that we've reset the buffer, there should be enough space. It's possible that we still
+  // won't have enough space in a few rare cases, such as vertex shader line/point expansion with a
+  // ton of lines in one draw command, in which case we will either need to add support for
+  // splitting a single draw command into multiple draws or using bigger indices.
+  ASSERT_MSG(VIDEO, count <= m_index_generator.GetRemainingIndices(primitive),
+             "VertexManager: Too few remaining index values ({} > {}). "
+             "32-bit indices or primitive breaking needed.",
+             count, m_index_generator.GetRemainingIndices(primitive));
+  ASSERT_MSG(VIDEO, count <= GetRemainingIndices(primitive),
+             "VertexManager: Buffer not large enough for all indices! ({} > {}) "
+             "Increase MAXIBUFFERSIZE or we need primitive breaking after all.",
+             count, GetRemainingIndices(primitive));
+  ASSERT_MSG(VIDEO, needed_vertex_bytes <= GetRemainingSize(),
+             "VertexManager: Buffer not large enough for all vertices! ({} > {}) "
+             "Increase MAXVBUFFERSIZE or we need primitive breaking after all.",
+             needed_vertex_bytes, GetRemainingSize());
+
   return DataReader(m_cur_buffer_pointer, m_end_buffer_pointer);
 }