diff --git a/repo2docker/app.py b/repo2docker/app.py
index 71171405..5feba05a 100755
--- a/repo2docker/app.py
+++ b/repo2docker/app.py
@@ -267,7 +267,7 @@ def _platform_default(self):
         """
         p = get_platform()
         if p == "linux/arm64":
-            warnings.warn(
+            self.log.warning(
                 "Building for linux/arm64 is experimental. "
                 "To use the recommended platform set --Repo2Docker.platform=linux/amd64. "
                 "To silence this warning set --Repo2Docker.platform=linux/arm64."
diff --git a/repo2docker/buildpacks/conda/__init__.py b/repo2docker/buildpacks/conda/__init__.py
index 2a3e694d..a9c6670e 100644
--- a/repo2docker/buildpacks/conda/__init__.py
+++ b/repo2docker/buildpacks/conda/__init__.py
@@ -355,7 +355,7 @@ def uses_r(self):
     @property
     def py2(self):
         """Am I building a Python 2 kernel environment?"""
-        warnings.warn(
+        self.log.warning(
             "CondaBuildPack.py2 is deprecated in 2023.2. Use CondaBuildPack.separate_kernel_env.",
             DeprecationWarning,
             stacklevel=2,
diff --git a/repo2docker/utils.py b/repo2docker/utils.py
index 9c2769e1..443db97e 100644
--- a/repo2docker/utils.py
+++ b/repo2docker/utils.py
@@ -543,5 +543,4 @@ def get_platform():
         # OSX reports arm64
         return "linux/arm64"
     else:
-        warnings.warn(f"Unexpected platform '{m}', defaulting to linux/amd64")
-        return "linux/amd64"
+        raise ValueError("Unsupported platform {m}")