diff --git a/qlib/FixedLengthUtil.qm b/qlib/FixedLengthUtil.qm
index 48688be62d..71380f8edf 100644
--- a/qlib/FixedLengthUtil.qm
+++ b/qlib/FixedLengthUtil.qm
@@ -237,6 +237,8 @@ public class FixedLengthUtil::FixedLengthAbstractIterator {
     /**
         @param spec Fixed-length line specification; see @ref fixedlengthspec for more information
         @param opts Global options; see @ref fixedlengthglobals for more information
+
+        @throw FIXED-LENGTH-UTIL-INVALID-SPEC invalid record specification; invalid type or missing field length
     */
     constructor(hash spec, *hash opts) {
         m_specs           = spec;
@@ -245,6 +247,8 @@ public class FixedLengthUtil::FixedLengthAbstractIterator {
         # Determine line lengths of each spec, for use by the default identifyType() method. We do this by building a reverse length -> type table.
         m_line_lengths = hash();
         foreach string k in (m_specs.keyIterator()) {
+            if (m_specs{k}.typeCode() != NT_HASH)
+                throw "FIXED-LENGTH-UTIL-INVALID-SPEC", sprintf("expecting a record description hash assigned to record key %y; got type %y instead (value: %y)", k, m_specs{k}.type(), m_specs{k});
             int len = 0;
             foreach string c in (m_specs{k}.keyIterator()) {
                 if (!m_specs{k}{c}.length)