diff --git a/include/protozero/basic_pbf_writer.hpp b/include/protozero/basic_pbf_writer.hpp
index 9e0af4ea..266e82ac 100644
--- a/include/protozero/basic_pbf_writer.hpp
+++ b/include/protozero/basic_pbf_writer.hpp
@@ -75,6 +75,8 @@ class basic_pbf_writer {
 
     // This is usually 0. If there is an open submessage, this is set in the
     // parent to the position where the data of the submessage is written to.
+    // If the size of the submessage was known when it was opened, this is
+    // set to the position where the data of the submessage must end instead.
     std::size_t m_pos = 0;
 
     void add_varint(uint64_t value) {
@@ -179,12 +181,13 @@ class basic_pbf_writer {
             m_rollback_pos = buffer_customization<TBuffer>::size(m_data);
             add_field(tag, pbf_wire_type::length_delimited);
             buffer_customization<TBuffer>::append_zeros(m_data, std::size_t(reserve_bytes));
+            m_pos = buffer_customization<TBuffer>::size(m_data);
         } else {
             m_rollback_pos = size_is_known;
             add_length_varint(tag, static_cast<pbf_length_type>(size));
             reserve(size);
+            m_pos = buffer_customization<TBuffer>::size(m_data) + size;
         }
-        m_pos = buffer_customization<TBuffer>::size(m_data);
     }
 
     void rollback_submessage() {
@@ -210,7 +213,14 @@ class basic_pbf_writer {
 
     void close_submessage() {
         protozero_assert(m_data);
-        if (m_pos == 0 || m_rollback_pos == size_is_known) {
+        if (m_pos == 0) {
+            return;
+        }
+        if (m_rollback_pos == size_is_known) {
+            const auto end_pos = m_pos;
+            m_pos = 0;
+            protozero_assert(buffer_customization<TBuffer>::size(m_data) == end_pos && "submessage with known size must have exactly that many bytes written");
+            (void)end_pos;
             return;
         }
         if (buffer_customization<TBuffer>::size(m_data) - m_pos == 0) {
diff --git a/test/t/repeated_packed_fixed32/reader_test_cases.cpp b/test/t/repeated_packed_fixed32/reader_test_cases.cpp
index fc2a179d..6bfdda0d 100644
--- a/test/t/repeated_packed_fixed32/reader_test_cases.cpp
+++ b/test/t/repeated_packed_fixed32/reader_test_cases.cpp
@@ -7,3 +7,56 @@ using cpp_type = uint32_t;
 
 #include <packed_access.hpp>
 
+
+TEST_CASE("write repeated packed fixed32 field using packed field with known size") {
+    std::string buffer;
+    protozero::pbf_writer pw{buffer};
+
+    SECTION("correct number of elements") {
+        {
+            protozero::packed_field_fixed32 field{pw, 1, 2};
+            field.add_element(17);
+            field.add_element(1);
+        }
+        pw.add_uint32(2, 5);
+
+        REQUIRE(buffer == std::string{"\x0a\x08\x11\x00\x00\x00\x01\x00\x00\x00\x10\x05", 12});
+    }
+
+    SECTION("correct number of elements with commit") {
+        protozero::packed_field_fixed32 field{pw, 1, 2};
+        field.add_element(17);
+        field.add_element(1);
+        field.commit();
+        pw.add_uint32(2, 5);
+
+        REQUIRE(buffer == std::string{"\x0a\x08\x11\x00\x00\x00\x01\x00\x00\x00\x10\x05", 12});
+    }
+
+    SECTION("too few elements") {
+        protozero::packed_field_fixed32 field{pw, 1, 2};
+        field.add_element(17);
+        REQUIRE_THROWS_AS(field.commit(), assert_error);
+    }
+
+    SECTION("too many elements") {
+        protozero::packed_field_fixed32 field{pw, 1, 2};
+        field.add_element(17);
+        field.add_element(1);
+        field.add_element(2);
+        REQUIRE_THROWS_AS(field.commit(), assert_error);
+    }
+}
+
+TEST_CASE("write submessage with known size") {
+    std::string buffer;
+    protozero::pbf_writer pw{buffer};
+
+    {
+        protozero::pbf_writer sw{pw, 1, 2};
+        sw.add_uint32(1, 5);
+    }
+    pw.add_uint32(2, 5);
+
+    REQUIRE(buffer == std::string{"\x0a\x02\x08\x05\x10\x05", 6});
+}