diff --git a/src/main/java/org/cojen/tupl/core/TupleKey.java b/src/main/java/org/cojen/tupl/core/TupleKey.java
index aacb6295e..b78d47d75 100644
--- a/src/main/java/org/cojen/tupl/core/TupleKey.java
+++ b/src/main/java/org/cojen/tupl/core/TupleKey.java
@@ -133,7 +133,7 @@ public abstract static class Maker {
 
         public abstract TupleKey with(Object[] e0, Object e1);
 
-        public abstract TupleKey with(Object[] e0, Object[] e1);
+        public abstract TupleKey with(Object[] e0);
 
         static CallSite indyWith(MethodHandles.Lookup lookup, String name, MethodType mt) {
             MethodMaker mm = MethodMaker.begin(lookup, name, mt);
diff --git a/src/main/java/org/cojen/tupl/table/join/JoinSpec.java b/src/main/java/org/cojen/tupl/table/join/JoinSpec.java
index 746c4abed..a498aa908 100644
--- a/src/main/java/org/cojen/tupl/table/join/JoinSpec.java
+++ b/src/main/java/org/cojen/tupl/table/join/JoinSpec.java
@@ -664,14 +664,18 @@ void appendTo(StringBuilder b) {
     public static sealed class Column extends Node implements Source {
         private final Table mTable;
         private final ColumnInfo mColumn;
+        private final boolean mDefined;
 
         private boolean mNullable;
 
-        Column(Table table, ColumnInfo column, boolean nullable) {
+        Column(Table table, ColumnInfo column, boolean defined, boolean nullable) {
             mTable = table;
             mColumn = column;
+            mDefined = defined;
             mNullable = nullable;
-            nullableCheck();
+            if (!defined) {
+                nullableCheck();
+            }
         }
 
         @Override
@@ -824,8 +828,14 @@ boolean isEmpty() throws IOException {
 
         @Override
         void nullable() {
-            mNullable = true;
-            nullableCheck();
+            if (!mNullable) {
+                mNullable = true;
+                if (mDefined) {
+                    mColumn.typeCode |= ColumnInfo.TYPE_NULLABLE;
+                } else {
+                    nullableCheck();
+                }
+            }
         }
 
         @Override
@@ -864,7 +874,7 @@ public static final class PlannedColumn extends Column implements Cloneable {
         private long mFilterScore;
 
         PlannedColumn(Column column) {
-            super(column.mTable, column.mColumn, column.mNullable);
+            super(column.mTable, column.mColumn, column.mDefined, column.mNullable);
         }
 
         @Override
@@ -1529,6 +1539,7 @@ private Node parseSource(boolean nullable) throws IOException {
             String name = mText.substring(start, --mPos);
 
             ColumnInfo column;
+            boolean defined;
 
             if (mAllColumns != null) {
                 column = mAllColumns.get(name);
@@ -1536,6 +1547,7 @@ private Node parseSource(boolean nullable) throws IOException {
                     mPos = start;
                     throw error("Unknown column");
                 }
+                defined = false;
             } else {
                 if (mNumTables >= mTables.length) {
                     throw new IllegalArgumentException("Not enough tables provided");
@@ -1544,6 +1556,7 @@ private Node parseSource(boolean nullable) throws IOException {
                 column.name = name;
                 column.type = mTables[mNumTables].rowType();
                 column.typeCode = ColumnInfo.TYPE_REFERENCE;
+                defined = true;
             }
 
             if (mSelectedColumns.putIfAbsent(name, column) != null) {
@@ -1568,7 +1581,7 @@ private Node parseSource(boolean nullable) throws IOException {
 
             mNumTables++;
 
-            return new Column(table, column, nullable);
+            return new Column(table, column, defined, nullable);
         }
 
         /**
diff --git a/src/main/java/org/cojen/tupl/table/join/JoinTableMaker.java b/src/main/java/org/cojen/tupl/table/join/JoinTableMaker.java
index 66cef088a..e1b9d629e 100644
--- a/src/main/java/org/cojen/tupl/table/join/JoinTableMaker.java
+++ b/src/main/java/org/cojen/tupl/table/join/JoinTableMaker.java
@@ -87,8 +87,12 @@ private static Class<Row> makeJoinTypeClass(Map<String, ColumnInfo> columns) {
         cm.sourceFile(JoinTableMaker.class.getSimpleName());
 
         for (ColumnInfo ci : columns.values()) {
-            cm.addMethod(ci.type, ci.name).public_().abstract_()
-                .addAnnotation(Nullable.class, true);
+            MethodMaker mm = cm.addMethod(ci.type, ci.name).public_().abstract_();
+
+            if (ci.isNullable()) {
+                mm.addAnnotation(Nullable.class, true);
+            }
+
             cm.addMethod(null, ci.name, ci.type).public_().abstract_();
         }
 
@@ -112,16 +116,20 @@ public static JoinTable<Row> join(String specStr, Table<?>... tables) {
 
         TupleKey cacheKey;
         {
-            var names = new String[definedColumns.size()];
-            var types = new Class[names.length];
+            var pairs = new Object[definedColumns.size() * 2];
             int i = 0;
             for (ColumnInfo column : definedColumns.values()) {
-                names[i] = column.name;
-                types[i] = column.type;
-                i++;
+                String name = column.name.intern();
+                if (column.isNullable()) {
+                    pairs[i++] = name;
+                    pairs[i++] = column.type;
+                } else {
+                    pairs[i++] = column.type;
+                    pairs[i++] = name;
+                }
             }
-            assert i == names.length;
-            cacheKey = TupleKey.make.with(names, types, definedColumns);
+            assert i == pairs.length;
+            cacheKey = TupleKey.make.with(pairs);
         }
 
         return join(cTypeCache.obtain(cacheKey, definedColumns), spec);