diff --git a/src/analyze/analyzed_ruby.c b/src/analyze/analyzed_ruby.c
index 30f534186..52794bffc 100644
--- a/src/analyze/analyzed_ruby.c
+++ b/src/analyze/analyzed_ruby.c
@@ -48,22 +48,22 @@ void free_analyzed_ruby(analyzed_ruby_T* analyzed) {
   free(analyzed);
 }
 
-const char* erb_keyword_from_analyzed_ruby(const analyzed_ruby_T* analyzed) {
+hb_string_T erb_keyword_from_analyzed_ruby(const analyzed_ruby_T* analyzed) {
   if (analyzed->end_count > 0) {
-    return "`<% end %>`";
+    return hb_string("`<% end %>`");
   } else if (analyzed->else_node_count > 0) {
-    return "`<% else %>`";
+    return hb_string("`<% else %>`");
   } else if (analyzed->elsif_node_count > 0) {
-    return "`<% elsif %>`";
+    return hb_string("`<% elsif %>`");
   } else if (analyzed->when_node_count > 0) {
-    return "`<% when %>`";
+    return hb_string("`<% when %>`");
   } else if (analyzed->in_node_count > 0) {
-    return "`<% in %>`";
+    return hb_string("`<% in %>`");
   } else if (analyzed->rescue_node_count > 0) {
-    return "`<% rescue %>`";
+    return hb_string("`<% rescue %>`");
   } else if (analyzed->ensure_node_count > 0) {
-    return "`<% ensure %>`";
+    return hb_string("`<% ensure %>`");
   }
 
-  return NULL;
+  return HB_STRING_NULL;
 }
diff --git a/src/analyze/invalid_structures.c b/src/analyze/invalid_structures.c
index 4681553e3..0a765f62a 100644
--- a/src/analyze/invalid_structures.c
+++ b/src/analyze/invalid_structures.c
@@ -49,15 +49,15 @@ bool detect_invalid_erb_structures(const AST_NODE_T* node, void* data) {
         return true;
       }
 
-      const char* keyword = NULL;
+      hb_string_T keyword = HB_STRING_NULL;
 
       if (context->loop_depth == 0) {
         if (has_error_message(analyzed, "Invalid break")) {
-          keyword = "`<% break %>`";
+          keyword = hb_string("`<% break %>`");
         } else if (has_error_message(analyzed, "Invalid next")) {
-          keyword = "`<% next %>`";
+          keyword = hb_string("`<% next %>`");
         } else if (has_error_message(analyzed, "Invalid redo")) {
-          keyword = "`<% redo %>`";
+          keyword = hb_string("`<% redo %>`");
         }
       } else {
         if (has_error_message(analyzed, "Invalid redo") || has_error_message(analyzed, "Invalid break")
@@ -71,7 +71,7 @@ bool detect_invalid_erb_structures(const AST_NODE_T* node, void* data) {
       }
 
       if (context->rescue_depth == 0) {
-        if (has_error_message(analyzed, "Invalid retry without rescue")) { keyword = "`<% retry %>`"; }
+        if (has_error_message(analyzed, "Invalid retry without rescue")) { keyword = hb_string("`<% retry %>`"); }
       } else {
         if (has_error_message(analyzed, "Invalid retry without rescue")) {
           if (is_loop_node) { context->loop_depth--; }
@@ -81,11 +81,11 @@ bool detect_invalid_erb_structures(const AST_NODE_T* node, void* data) {
         }
       }
 
-      if (keyword == NULL) { keyword = erb_keyword_from_analyzed_ruby(analyzed); }
+      if (hb_string_is_null(keyword)) { keyword = erb_keyword_from_analyzed_ruby(analyzed); }
 
-      if (keyword != NULL && !token_value_empty(content_node->tag_closing)) {
+      if (!hb_string_is_null(keyword) && !token_value_empty(content_node->tag_closing)) {
         append_erb_control_flow_scope_error(
-          hb_string(keyword),
+          keyword,
           node->location.start,
           node->location.end,
           context->allocator,
@@ -116,11 +116,11 @@ bool detect_invalid_erb_structures(const AST_NODE_T* node, void* data) {
 
         if (content_node->parsed && !content_node->valid && content_node->analyzed_ruby != NULL) {
           analyzed_ruby_T* analyzed = content_node->analyzed_ruby;
-          const char* keyword = erb_keyword_from_analyzed_ruby(analyzed);
+          hb_string_T keyword = erb_keyword_from_analyzed_ruby(analyzed);
 
           if (!token_value_empty(content_node->tag_closing)) {
             append_erb_control_flow_scope_error(
-              hb_string(keyword),
+              keyword,
               subsequent->location.start,
               subsequent->location.end,
               context->allocator,
diff --git a/src/include/analyze/analyzed_ruby.h b/src/include/analyze/analyzed_ruby.h
index 12387c755..25b12ca8a 100644
--- a/src/include/analyze/analyzed_ruby.h
+++ b/src/include/analyze/analyzed_ruby.h
@@ -35,6 +35,6 @@ typedef struct ANALYZED_RUBY_STRUCT {
 
 analyzed_ruby_T* init_analyzed_ruby(hb_string_T source);
 void free_analyzed_ruby(analyzed_ruby_T* analyzed);
-const char* erb_keyword_from_analyzed_ruby(const analyzed_ruby_T* analyzed);
+hb_string_T erb_keyword_from_analyzed_ruby(const analyzed_ruby_T* analyzed);
 
 #endif
diff --git a/src/include/prism_helpers.h b/src/include/prism_helpers.h
index 62767b984..3c7c9453f 100644
--- a/src/include/prism_helpers.h
+++ b/src/include/prism_helpers.h
@@ -10,7 +10,7 @@
 
 #include <prism.h>
 
-const char* pm_error_level_to_string(pm_error_level_t level);
+hb_string_T pm_error_level_to_string(pm_error_level_t level);
 
 RUBY_PARSE_ERROR_T* ruby_parse_error_from_prism_error(
   const pm_diagnostic_t* error,
diff --git a/src/prism_helpers.c b/src/prism_helpers.c
index f9a310bb0..46fde9d85 100644
--- a/src/prism_helpers.c
+++ b/src/prism_helpers.c
@@ -10,13 +10,11 @@
 #include <stdlib.h>
 #include <string.h>
 
-const char* pm_error_level_to_string(pm_error_level_t level) {
+hb_string_T pm_error_level_to_string(pm_error_level_t level) {
   switch (level) {
-    case PM_ERROR_LEVEL_SYNTAX: return "syntax";
-    case PM_ERROR_LEVEL_ARGUMENT: return "argument";
-    case PM_ERROR_LEVEL_LOAD: return "load";
-
-    default: return "Unknown pm_error_level_t";
+    case PM_ERROR_LEVEL_SYNTAX: return hb_string("syntax");
+    case PM_ERROR_LEVEL_ARGUMENT: return hb_string("argument");
+    case PM_ERROR_LEVEL_LOAD: return hb_string("load");
   }
 }
 
@@ -36,7 +34,7 @@ RUBY_PARSE_ERROR_T* ruby_parse_error_from_prism_error(
   return ruby_parse_error_init(
     hb_string(error->message),
     hb_string(pm_diagnostic_id_human(error->diag_id)),
-    hb_string(pm_error_level_to_string(error->level)),
+    pm_error_level_to_string(error->level),
     start,
     end,
     allocator
@@ -52,7 +50,7 @@ RUBY_PARSE_ERROR_T* ruby_parse_error_from_prism_error_with_positions(
   return ruby_parse_error_init(
     hb_string(error->message),
     hb_string(pm_diagnostic_id_human(error->diag_id)),
-    hb_string(pm_error_level_to_string(error->level)),
+    pm_error_level_to_string(error->level),
     start,
     end,
     allocator