diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 86a8e27..98d4b04 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -36,6 +36,16 @@ jobs:
       with:
         ruby-version: ${{ matrix.ruby }}
         bundler-cache: true # 'bundle install' and cache
+    - run: env | sort
+    - run: |
+        cc -xc -E - <<EOF
+        #include <dlfcn.h>
+        foo(RTLD_LAZY, RTLD_NOW, RTLD_LOCAL, RTLD_GLOBAL);
+        EOF
     - name: Run test
       run: bundle exec rake compile test
       timeout-minutes: 5
+      env:
+        TESTOPTS: "--verbose"
+        TRUFFLERUBYOPT: "--experimental-options --cexts-log-load"
+        MACOSX_DEPLOYMENT_TARGET: "11.0"
diff --git a/ext/zlib/zlib.c b/ext/zlib/zlib.c
index dc608ee..8556b48 100644
--- a/ext/zlib/zlib.c
+++ b/ext/zlib/zlib.c
@@ -2965,6 +2965,26 @@ gzfile_read_all(struct gzfile *gz)
     return gzfile_newstr(gz, dst);
 }
 
+__attribute__((__noinline__))
+static VALUE
+gzfile_getc_dummy_encoding(struct gzfile *gz)
+{
+    const unsigned char *ss, *sp, *se;
+    unsigned char *ds, *dp, *de;
+    VALUE cbuf = rb_enc_str_new(0, GZFILE_CBUF_CAPA, gz->enc);
+
+    ss = sp = (const unsigned char*)RSTRING_PTR(gz->z.buf);
+    se = sp + ZSTREAM_BUF_FILLED(&gz->z);
+    ds = dp = (unsigned char *)RSTRING_PTR(cbuf);
+    de = (unsigned char *)ds + GZFILE_CBUF_CAPA;
+    (void)rb_econv_convert(gz->ec, &sp, se, &dp, de, ECONV_PARTIAL_INPUT|ECONV_AFTER_OUTPUT);
+    rb_econv_check_error(gz->ec);
+    VALUE dst = zstream_shift_buffer(&gz->z, sp - ss);
+    gzfile_calc_crc(gz, dst);
+    rb_str_resize(cbuf, dp - ds);
+    return cbuf;
+}
+
 static VALUE
 gzfile_getc(struct gzfile *gz)
 {
@@ -2983,20 +3003,7 @@ gzfile_getc(struct gzfile *gz)
     }
 
     if (gz->ec && rb_enc_dummy_p(gz->enc2)) {
-	const unsigned char *ss, *sp, *se;
-	unsigned char *ds, *dp, *de;
-	VALUE cbuf = rb_enc_str_new(0, GZFILE_CBUF_CAPA, gz->enc);
-
-        ss = sp = (const unsigned char*)RSTRING_PTR(gz->z.buf);
-        se = sp + ZSTREAM_BUF_FILLED(&gz->z);
-        ds = dp = (unsigned char *)RSTRING_PTR(cbuf);
-        de = (unsigned char *)ds + GZFILE_CBUF_CAPA;
-        (void)rb_econv_convert(gz->ec, &sp, se, &dp, de, ECONV_PARTIAL_INPUT|ECONV_AFTER_OUTPUT);
-        rb_econv_check_error(gz->ec);
-	dst = zstream_shift_buffer(&gz->z, sp - ss);
-	gzfile_calc_crc(gz, dst);
-	rb_str_resize(cbuf, dp - ds);
-	return cbuf;
+        return gzfile_getc_dummy_encoding(gz);
     }
     else {
 	buf = gz->z.buf;