diff --git a/include/seqan3/io/stream/detail/fast_istreambuf_iterator.hpp b/include/seqan3/io/stream/detail/fast_istreambuf_iterator.hpp
index 78a61cb145..9032e8903a 100644
--- a/include/seqan3/io/stream/detail/fast_istreambuf_iterator.hpp
+++ b/include/seqan3/io/stream/detail/fast_istreambuf_iterator.hpp
@@ -69,7 +69,9 @@ class fast_istreambuf_iterator
         stream_buf{reinterpret_cast<stream_buffer_exposer<char_t, traits_t> *>(&ibuf)}
     {
         assert(stream_buf != nullptr);
-        stream_buf->underflow(); // ensure the stream buffer has content on construction
+        if (!stream_buf->in_avail())
+            stream_buf->underflow(); // ensure the stream buffer has content on construction
+        assert(stream_buf.in_avail());
     }
     //!\}
 
diff --git a/test/unit/io/sam_file/sam_file_format_test_template.hpp b/test/unit/io/sam_file/sam_file_format_test_template.hpp
index 6a07e4a9fe..08202042b3 100644
--- a/test/unit/io/sam_file/sam_file_format_test_template.hpp
+++ b/test/unit/io/sam_file/sam_file_format_test_template.hpp
@@ -223,31 +223,23 @@ TYPED_TEST_P(sam_file_read, read_in_all_data)
 
 class null_filter_buf : public std::streambuf
 {
+    static constexpr std::streamsize buffer_size{10};
     std::streambuf * src;
-    std::array<char, 10> bufl{};
+    std::array<char, buffer_size> bufl{};
 
 public:
-
-    null_filter_buf() = delete;                               //!< Defaulted.
-    null_filter_buf(null_filter_buf const &) = delete;             //!< Defaulted.
-    null_filter_buf & operator=(null_filter_buf const &) = delete; //!< Defaulted.
-    null_filter_buf(null_filter_buf &&) = delete;                  //!< Defaulted.
-    null_filter_buf & operator=(null_filter_buf &&) = delete;      //!< Defaulted.
-    ~null_filter_buf() = default;                              //!< Defaulted.
-
     int underflow()
     {
-        std::streamsize number_of_characters_read = src->sgetn(bufl.data(), 10);
+        std::streamsize number_of_characters_read = src->sgetn(bufl.data(), buffer_size);
 
         setg(bufl.data(), bufl.data(), bufl.data() + number_of_characters_read); // make read position available
 
-        return (number_of_characters_read == 10) ? *bufl.data() : traits_type::eof();
+        return (number_of_characters_read == buffer_size) ? *bufl.data() : traits_type::eof();
     }
 
-    null_filter_buf(std::streambuf* buf) : src(buf)
+    null_filter_buf(std::streambuf * buf) : src(buf)
     {
-        std::cout << "I'm getting consstructed and I'm filling my buffer" << std::endl;
-        underflow();
+        setg(bufl.data(), bufl.data(), bufl.data() + buffer_size);
     }
 };