diff --git a/src/rntuple.cpp b/src/rntuple.cpp
index 2f13afb..d78c0e6 100644
--- a/src/rntuple.cpp
+++ b/src/rntuple.cpp
@@ -535,6 +535,8 @@ String8_Node *display_val_rootzip(Arena *arena, String8_Node *prev, const char *
   }
   if (src[0] == 'Z' && src[1] == 'S' && src[2] == 1) {
     zip_method = str8("ZSTD");
+  } else {
+    return nullptr;
   }
   u32 comp_size = src[3] | (src[4] << 8) | (src[5] << 16);
   u32 uncomp_size = src[6] | (src[7] << 8) | (src[8] << 16);
@@ -547,6 +549,9 @@ String8_Node *display_val_rootzip(Arena *arena, String8_Node *prev, const char *
   return sn;
 }
 
+template <typename T>
+using Display_Fn = String8_Node *(*)(Arena *, String8_Node *, const char *, T);
+
 // Functor used by get_section_hover_info to describe the structure of a section and print data about it.
 struct Try_Sec_Hover_Fn {
   u64 start;
@@ -556,12 +561,10 @@ struct Try_Sec_Hover_Fn {
   Sec_Hover_Info &info;
   u64 &cur_field_off;
 
-  template <typename TField_Type>
-  b8 field(const char *desc_fmt,
-           String8_Node *(*display_val)(Arena *, String8_Node *, const char *, TField_Type) = hover_display_val_be<TField_Type>
-           ) const
+  template <typename T>
+  b8 field(const char *desc_fmt, Display_Fn<T> display_val = hover_display_val_be<T>) const
   {
-    u64 field_len = sizeof(TField_Type);
+    u64 field_len = sizeof(T);
     if (roff < cur_field_off + field_len) {
       info.rng = { start + cur_field_off, field_len };
       TField_Type val;
@@ -574,9 +577,7 @@ struct Try_Sec_Hover_Fn {
   }
 
   template <>
-  b8 field<String8>(const char *desc_fmt,
-                    String8_Node *(*display_val)(Arena *, String8_Node *, const char *, String8)
-                    ) const
+  b8 field<String8>(const char *desc_fmt, Display_Fn<String8> display_val) const
   {
     u8 str_size = data[start + cur_field_off];
     if (roff < cur_field_off + 1 + str_size) {
@@ -603,14 +604,11 @@ struct Try_Sec_Hover_Fn {
     return false;
   }
 
-  b8 range_data(const char *desc, u64 range_len,
-                String8_Node *(*display_val)(Arena *, String8_Node *, const char *, const u8 *)
-                ) const
+  b8 range_data(const char *desc, u64 range_len, Display_Fn<const u8 *> display_val) const
   {
     if (roff < cur_field_off + range_len) {
       info.rng = { start + cur_field_off, range_len };
-      display_val(arena, info.desc, desc, data + start + cur_field_off);
-      return true;
+      return display_val(arena, info.desc, desc, data + start + cur_field_off);
     }
     cur_field_off += range_len;
     return false;
@@ -663,10 +661,9 @@ b8 hover_try_key(const Try_Sec_Hover_Fn &try_sec_hover, const u8 *data, u64 star
 }
 
 internal
-b8 hover_try_rootzip(const Try_Sec_Hover_Fn &try_sec_hover, const u8 *data, u64 start)
+b8 hover_try_rootzip(const Try_Sec_Hover_Fn &try_sec_hover, const u8 *, u64)
 {
   // TODO boundary checks
-
   return try_sec_hover.range_data("Zipped Block", 9, display_val_rootzip);
 }