diff --git a/mpact/sim/decoder/instruction_set_visitor.cc b/mpact/sim/decoder/instruction_set_visitor.cc
index 0374561..acd112a 100644
--- a/mpact/sim/decoder/instruction_set_visitor.cc
+++ b/mpact/sim/decoder/instruction_set_visitor.cc
@@ -2053,7 +2053,7 @@
     if (num.empty()) {
       delete format_info;
       return absl::InternalError(
-          absl::StrCat("Malformed expression '", expr, "'"));
+          absl::StrCat("Malformed expression - no shift amount '", expr, "'"));
     }
     format_info->shift_amount = std::stoi(num);
 
@@ -2063,13 +2063,14 @@
     if (expr[pos] != ')') {
       delete format_info;
       return absl::InternalError(
-          absl::StrCat("Malformed expression '", expr, "'"));
+          absl::StrCat("Malformed expression - expected ')' '", expr, "'"));
     }
+    pos++;
     pos = skip_space(expr, pos);
     if (pos != std::string::npos) {
       delete format_info;
-      return absl::InternalError(
-          absl::StrCat("Malformed expression '", expr, "'"));
+      return absl::InternalError(absl::StrCat(
+          "Malformed expression - extra characters after ')' '", expr, "'"));
     }
   }
   return format_info;
diff --git a/mpact/sim/decoder/test/instruction_set_visitor_test.cc b/mpact/sim/decoder/test/instruction_set_visitor_test.cc
index c1d7623..d5b0e74 100644
--- a/mpact/sim/decoder/test/instruction_set_visitor_test.cc
+++ b/mpact/sim/decoder/test/instruction_set_visitor_test.cc
@@ -504,16 +504,18 @@
   ptr = log_sink.error_log().find(
       "Error: Missing shift in expression '@+(rs1 +-)'");
   EXPECT_TRUE(ptr != std::string::npos);
-  ptr =
-      log_sink.error_log().find("Error: Malformed expression '@+(rs1 << 5x)'");
+  ptr = log_sink.error_log().find(
+      "Error: Malformed expression - expected ')' '@+(rs1 << 5x)'");
   EXPECT_TRUE(ptr != std::string::npos);
-  ptr = log_sink.error_log().find("Error: Malformed expression '@+(rs1 >> )'");
+  ptr = log_sink.error_log().find(
+      "Error: Malformed expression - no shift amount '@+(rs1 >> )'");
   EXPECT_TRUE(ptr != std::string::npos);
-  ptr =
-      log_sink.error_log().find("Error: Malformed expression '@+(rs1 << 5 x)'");
+  ptr = log_sink.error_log().find(
+      "Error: Malformed expression - expected ')' '@+(rs1 << 5 x)'");
   EXPECT_TRUE(ptr != std::string::npos);
-  ptr =
-      log_sink.error_log().find("Error: Malformed expression '@+(rs1 << 5) x'");
+  ptr = log_sink.error_log().find(
+      "Error: Malformed expression - extra characters after ')' '@+(rs1 << 5) "
+      "x'");
   EXPECT_TRUE(ptr != std::string::npos);
   ptr = log_sink.error_log().find(
       "Error: Format width required when a leading 0 is specified - '0x'");