diff --git a/python/sglang/srt/configs/model_config.py b/python/sglang/srt/configs/model_config.py
index 6483487fd..33938ff48 100644
--- a/python/sglang/srt/configs/model_config.py
+++ b/python/sglang/srt/configs/model_config.py
@@ -358,9 +358,10 @@ class ModelConfig:
                 mscale_all_dim = self.hf_config.rope_scaling.get(
                     "mscale_all_dim", False
                 )
-                scaling_factor = self.hf_config.rope_scaling["factor"]
-                mscale = yarn_get_mscale(scaling_factor, float(mscale_all_dim))
-                self.scaling = self.scaling * mscale * mscale
+                scaling_factor = self.hf_config.rope_scaling.get("factor")
+                if scaling_factor is not None:
+                    mscale = yarn_get_mscale(scaling_factor, float(mscale_all_dim))
+                    self.scaling = self.scaling * mscale * mscale
 
         elif "MiniCPM3ForCausalLM" in self.hf_config.architectures:
             self.head_dim = 128