diff --git a/pglablib/ResultSourceMapping.cpp b/pglablib/ResultSourceMapping.cpp
index d597504..8197133 100644
--- a/pglablib/ResultSourceMapping.cpp
+++ b/pglablib/ResultSourceMapping.cpp
@@ -1,8 +1,12 @@
 #include "ResultSourceMapping.h"
 
 #include "Pgsql_Result.h"
+#include "catalog/PgClass.h"
 #include "catalog/PgDatabaseCatalog.h"
 #include "catalog/PgConstraintContainer.h"
+#include "catalog/PgNamespace.h"
+#include "catalog/PgNamespaceContainer.h"
+#include "catalog/PgClassContainer.h"
 
 #include <algorithm>
 #include <stdexcept>
@@ -107,6 +111,20 @@ const ResultSourceMapping::TableInfo *ResultSourceMapping::findTable(Oid tableOi
     return found == m_tables.end() ? nullptr : &*found;
 }
 
+namespace {
+
+/// Catalog tables (pg_catalog, information_schema, ...) are views or system tables and must never be edited.
+bool isSystemCatalogTable(const PgDatabaseCatalog &catalog, Oid tableOid)
+{
+    const PgClass *cls = catalog.classes()->getByKey(tableOid);
+    if (!cls)
+        return false;
+    const PgNamespace *ns = catalog.namespaces()->getByKey(cls->nsOid());
+    return ns && ns->isSystemCatalog();
+}
+
+}
+
 ResultSourceMapping createResultSourceMapping(const Pgsql::Result &result, const PgDatabaseCatalog &catalog)
 {
     std::vector<ResultColumnSource> columns;
@@ -129,9 +147,11 @@ ResultSourceMapping createResultSourceMapping(const Pgsql::Result &result, const
 
         TablePrimaryKey pk;
         pk.tableOid = source.tableOid;
-        auto constraint = catalog.constraints()->getPrimaryForRelation(source.tableOid);
-        if (constraint)
-            pk.attnums.assign(constraint->key.begin(), constraint->key.end());
+        if (!isSystemCatalogTable(catalog, source.tableOid)) {
+            auto constraint = catalog.constraints()->getPrimaryForRelation(source.tableOid);
+            if (constraint)
+                pk.attnums.assign(constraint->key.begin(), constraint->key.end());
+        }
         primaryKeys.push_back(pk);
     }
 
diff --git a/releasenotes/notes/query-results-editable-grid-b265a193a39f3245.yaml b/releasenotes/notes/query-results-editable-grid-b265a193a39f3245.yaml
index 14be544..a54b9f5 100644
--- a/releasenotes/notes/query-results-editable-grid-b265a193a39f3245.yaml
+++ b/releasenotes/notes/query-results-editable-grid-b265a193a39f3245.yaml
@@ -4,9 +4,10 @@ features:
     Query results are now shown in the same grid as the table data tab. Columns
     can be sorted by clicking their headers, and cells that belong to a table
     with a primary key present in the result can be edited directly. Changes
-    are saved with an ``UPDATE`` by primary key.
+    are saved with an ``UPDATE`` by primary key. Tables in system catalog
+    schemas such as ``pg_catalog`` and ``information_schema`` are never editable.
 other:
   - |
-    Read-only cells in query results, such as expression columns or columns of
-    tables without a primary key in the result, are now shown in grey so they
-    can be told apart from editable cells.
+    Read-only cells in query results, such as expression columns, columns of
+    tables without a primary key in the result, or columns of system catalog
+    tables, are now shown in grey so they can be told apart from editable cells.