diff --git a/csparser.cc b/csparser.cc
index 2d63f93c..ec00a782 100644
--- a/csparser.cc
+++ b/csparser.cc
@@ -118,7 +118,7 @@ class ErrFileLexer {
             reComment_("^(#)(.*)$"),
             reChecker_("^Error: *([A-Za-z][0-9A-Za-z_.]+)( *\\([^)]+\\))? *:$"),
             reEvent_(
-                    /* location */ "^([^:]+)(?::([0-9]+))?(?::([0-9]+))?"
+                    /* location */ "^([^:]+)(?::([0-9]+|<unknown>))?(?::([0-9]+))?"
                     /* evt/mesg */ ": (" RE_EVENT "): (.*)$")
         {
         }
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index 38e9c342..92186383 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -170,6 +170,7 @@ test_csgrep(csgrep      "50-gcc-parser-gcc-9.2.1"     )
 test_csgrep(csgrep      "51-gcc-parser-systemd"       )
 test_csgrep(csgrep      "52-gcc-parser-clang-nss"     )
 test_csgrep(csgrep      "53-csparser-note-events"     )
+test_csgrep(csgrep      "54-csparser-unknown-lineno"  )
 test_csparser(csparser-5.8                          00)
 test_csparser(csparser-5.8                          01)
 test_csparser(csparser-5.8                          02)
diff --git a/tests/csgrep/54-csparser-unknown-lineno-args.txt b/tests/csgrep/54-csparser-unknown-lineno-args.txt
new file mode 100644
index 00000000..e69de29b
diff --git a/tests/csgrep/54-csparser-unknown-lineno-stdin.txt b/tests/csgrep/54-csparser-unknown-lineno-stdin.txt
new file mode 100644
index 00000000..6d919caf
--- /dev/null
+++ b/tests/csgrep/54-csparser-unknown-lineno-stdin.txt
@@ -0,0 +1,7 @@
+Error: CBMC_WARNING:
+<unknown>:<unknown>: error: memory-leak : dynamically allocated memory never freed in __CPROVER_memory_leak == NULL
+one_memory_leak.c:3: note: from_entry_point_function:function-call:main
+one_memory_leak.c:4: note: main:4:pointer:        str = ((char *)NULL)
+one_memory_leak.c:4: note: from_main:function-call:malloc
+one_memory_leak.c:4: note: function_parameter_set_in:main:4:integer:        malloc_size = 15ul
+one_memory_leak.c:5: note: from_main:function-return:main
diff --git a/tests/csgrep/54-csparser-unknown-lineno-stdout.txt b/tests/csgrep/54-csparser-unknown-lineno-stdout.txt
new file mode 100644
index 00000000..994c14a9
--- /dev/null
+++ b/tests/csgrep/54-csparser-unknown-lineno-stdout.txt
@@ -0,0 +1,7 @@
+Error: CBMC_WARNING:
+<unknown>: error: memory-leak : dynamically allocated memory never freed in __CPROVER_memory_leak == NULL
+one_memory_leak.c:3: note: from_entry_point_function:function-call:main
+one_memory_leak.c:4: note: main:4:pointer:        str = ((char *)NULL)
+one_memory_leak.c:4: note: from_main:function-call:malloc
+one_memory_leak.c:4: note: function_parameter_set_in:main:4:integer:        malloc_size = 15ul
+one_memory_leak.c:5: note: from_main:function-return:main