diff --git a/core/src/smoke_gcode_parser.cpp b/core/src/smoke_gcode_parser.cpp
index a5aeb33..5c07a3e 100644
--- a/core/src/smoke_gcode_parser.cpp
+++ b/core/src/smoke_gcode_parser.cpp
@@ -1493,10 +1493,18 @@ std::string expand_expressions_and_parameters(const std::string &line,
     out.reserve(line.size());
     for (size_t i = 0; i < line.size();) {
         if (std::isalpha(static_cast<unsigned char>(line[i])) && i + 1 < line.size()) {
+            const size_t value_start = i + 1;
+            size_t function_start = value_start;
+            if ((line[function_start] == '+' || line[function_start] == '-') &&
+                function_start + 1 < line.size() &&
+                !std::isdigit(static_cast<unsigned char>(line[function_start + 1])) &&
+                line[function_start + 1] != '.') {
+                ++function_start;
+            }
             size_t function_end = 0;
-            if (unary_function_span(line, i + 1, &function_end)) {
+            if (unary_function_span(line, function_start, &function_end)) {
                 double value = 0.0;
-                if (!evaluate_expression(line.substr(i + 1, function_end - i - 1),
+                if (!evaluate_expression(line.substr(value_start, function_end - value_start),
                                          call_stack,
                                          parameters,
                                          named_parameters,
diff --git a/core/tests/cnc_sim_api_smoke.cpp b/core/tests/cnc_sim_api_smoke.cpp
index 4aa1922..19047ea 100644
--- a/core/tests/cnc_sim_api_smoke.cpp
+++ b/core/tests/cnc_sim_api_smoke.cpp
@@ -273,6 +273,7 @@ int main() {
     bool saw_math_function_assignment = false;
     bool saw_word_unary_function_motion = false;
     bool saw_word_atan_function_motion = false;
+    bool saw_signed_word_unary_function_motion = false;
     bool saw_trig_function_motion = false;
     bool saw_inverse_trig_function_motion = false;
     bool saw_atan_function_motion = false;
@@ -3571,7 +3572,8 @@ int main() {
         "G21 G90\n"
         "F100\n"
         "G1 XABS[-2]\n"
-        "G1 YATAN[1]/[1]\n";
+        "G1 YATAN[1]/[1]\n"
+        "G1 Z-ABS[-2]\n";
     events.clear();
     ok &= expect(cnc_sim_parse_program(sim,
                                        word_unary_function_program,
@@ -3586,9 +3588,14 @@ int main() {
                                         (event.type == CNC_SIM_EVENT_LINEAR_FEED &&
                                          event.line == 4 &&
                                          near(event.end.y, 45.0));
+        saw_signed_word_unary_function_motion = saw_signed_word_unary_function_motion ||
+                                                (event.type == CNC_SIM_EVENT_LINEAR_FEED &&
+                                                 event.line == 5 &&
+                                                 event.end.z == -2.0);
     }
     ok &= expect(saw_word_unary_function_motion, "expected LinuxCNC unary function as a word value");
     ok &= expect(saw_word_atan_function_motion, "expected LinuxCNC ATAN function as a word value");
+    ok &= expect(saw_signed_word_unary_function_motion, "expected signed LinuxCNC unary function as a word value");
 
     const char additional_math_function_program[] =
         "#1 = [ASIN[0.5] + ACOS[0.5]]\n"