diff --git a/spec/ruby/optional/capi/encoding_spec.rb b/spec/ruby/optional/capi/encoding_spec.rb
index dc1019e8ae..cd3f00d478 100644
--- a/spec/ruby/optional/capi/encoding_spec.rb
+++ b/spec/ruby/optional/capi/encoding_spec.rb
@@ -37,9 +37,9 @@ describe "C-API Encoding function" do
     @s = CApiEncodingSpecs.new
   end
 
-  describe "rb_enc_alias" do
+  describe "rb_encdb_alias" do
     it "creates an alias for an existing Encoding" do
-      @s.rb_enc_alias("ZOMGWTFBBQ", "UTF-8").should >= 0
+      @s.rb_encdb_alias("ZOMGWTFBBQ", "UTF-8").should >= 0
       Encoding.find("ZOMGWTFBBQ").name.should == "UTF-8"
     end
   end
diff --git a/spec/ruby/optional/capi/ext/encoding_spec.c b/spec/ruby/optional/capi/ext/encoding_spec.c
index 9ab893b86a..6f4057865b 100644
--- a/spec/ruby/optional/capi/ext/encoding_spec.c
+++ b/spec/ruby/optional/capi/ext/encoding_spec.c
@@ -93,9 +93,12 @@ static VALUE encoding_spec_rb_default_external_encoding(VALUE self) {
 }
 #endif
 
-#ifdef HAVE_RB_ENC_ALIAS
-static VALUE encoding_spec_rb_enc_alias(VALUE self, VALUE alias, VALUE orig) {
-  return INT2NUM(rb_enc_alias(RSTRING_PTR(alias), RSTRING_PTR(orig)));
+#ifdef HAVE_RB_ENCDB_ALIAS
+/* Not exposed by MRI C-API encoding.h but used in the pg gem. */
+extern int rb_encdb_alias(const char* alias, const char* orig);
+
+static VALUE encoding_spec_rb_encdb_alias(VALUE self, VALUE alias, VALUE orig) {
+  return INT2NUM(rb_encdb_alias(RSTRING_PTR(alias), RSTRING_PTR(orig)));
 }
 #endif
 
@@ -323,8 +326,8 @@ void Init_encoding_spec(void) {
                    encoding_spec_rb_default_external_encoding, 0);
 #endif
 
-#ifdef HAVE_RB_ENC_ALIAS
-  rb_define_method(cls, "rb_enc_alias", encoding_spec_rb_enc_alias, 2);
+#ifdef HAVE_RB_ENCDB_ALIAS
+  rb_define_method(cls, "rb_encdb_alias", encoding_spec_rb_encdb_alias, 2);
 #endif
 
 #ifdef HAVE_RB_ENC_ASSOCIATE
diff --git a/spec/ruby/optional/capi/ext/rubyspec.h b/spec/ruby/optional/capi/ext/rubyspec.h
index 1ece9541c7..4f50450dfb 100644
--- a/spec/ruby/optional/capi/ext/rubyspec.h
+++ b/spec/ruby/optional/capi/ext/rubyspec.h
@@ -204,7 +204,7 @@
 #define HAVE_RB_DEFAULT_INTERNAL_ENCODING  1
 #define HAVE_RB_DEFAULT_EXTERNAL_ENCODING  1
 
-#define HAVE_RB_ENC_ALIAS                  1
+#define HAVE_RB_ENCDB_ALIAS                1
 #define HAVE_RB_ENC_ASSOCIATE              1
 #define HAVE_RB_ENC_ASSOCIATE_INDEX        1
 #define HAVE_RB_ENC_CODEPOINT_LEN          1