diff --git a/src/libOpenImageIO/imageinput.cpp b/src/libOpenImageIO/imageinput.cpp
index 9d2d658ce7..baddc36e08 100644
--- a/src/libOpenImageIO/imageinput.cpp
+++ b/src/libOpenImageIO/imageinput.cpp
@@ -755,6 +755,8 @@ ImageInput::read_native_tile(int /*subimage*/, int /*miplevel*/, int /*x*/,
     // The base class read_native_tile fails. A format reader that supports
     // tiles MUST overload this virtual method that reads a single tile
     // (all channels).
+    errorfmt("ImageInput::read_native_tile call unimplemented for {}",
+             format_name());
     return false;
 }
 
diff --git a/src/tiff.imageio/tiffoutput.cpp b/src/tiff.imageio/tiffoutput.cpp
index 82a2b3350b..053ad690f3 100644
--- a/src/tiff.imageio/tiffoutput.cpp
+++ b/src/tiff.imageio/tiffoutput.cpp
@@ -1809,7 +1809,8 @@ TIFFOutput::fix_bitdepth(void* data, int nvals)
             v[i] = bit_range_convert<32, 24>(v[i]);
         bit_pack(cspan<unsigned int>(v, v + nvals), v, 24);
     } else {
-        OIIO_ASSERT(0 && "unsupported bit conversion -- shouldn't reach here");
+        errorfmt("unsupported bit conversion: {} -> {}", spec().format,
+                 m_bitspersample);
     }
 }