diff --git a/include/diagnostics.hpp b/include/diagnostics.hpp
index a529d2f7..6274d93f 100644
--- a/include/diagnostics.hpp
+++ b/include/diagnostics.hpp
@@ -28,7 +28,7 @@ struct WarningState {
 	void update(WarningState other);
 };
 
-std::pair<WarningState, std::optional<uint8_t>> getInitialWarningState(std::string &flag);
+std::pair<WarningState, std::optional<uint32_t>> getInitialWarningState(std::string &flag);
 
 template<typename L>
 struct WarningFlag {
@@ -150,20 +150,17 @@ std::string Diagnostics<L, W>::processWarningFlag(char const *flag) {
 		if (!param.has_value() || *param == 0) {
 			param = paramWarning.defaultLevel;
 		} else if (*param > maxParam) {
-			if (*param != 255) { // Don't warn if already capped
-				warnx(
-				    "Invalid parameter %" PRIu8
-				    " for warning flag \"%s\"; capping at maximum %" PRIu8,
-				    *param,
-				    rootFlag.c_str(),
-				    maxParam
-				);
-			}
+			warnx(
+			    "Invalid warning flag parameter \"%s=%" PRIu32 "\"; capping at maximum %" PRIu8,
+			    rootFlag.c_str(),
+			    *param,
+			    maxParam
+			);
 			*param = maxParam;
 		}
 
 		// Set the first <param> to enabled/error, and disable the rest
-		for (uint8_t ofs = 0; ofs < maxParam; ofs++) {
+		for (uint32_t ofs = 0; ofs < maxParam; ofs++) {
 			if (WarningState &warning = state.flagStates[baseID + ofs]; ofs < *param) {
 				warning.update(flagState);
 			} else {
@@ -174,7 +171,7 @@ std::string Diagnostics<L, W>::processWarningFlag(char const *flag) {
 	}
 
 	if (param.has_value()) {
-		warnx("Unknown warning flag parameter \"%s\"", rootFlag.c_str());
+		warnx("Unknown warning flag parameter \"%s=%" PRIu32 "\"", rootFlag.c_str(), *param);
 		return rootFlag;
 	}
 
diff --git a/src/diagnostics.cpp b/src/diagnostics.cpp
index f0df7cf0..c0a64a01 100644
--- a/src/diagnostics.cpp
+++ b/src/diagnostics.cpp
@@ -18,7 +18,7 @@ void WarningState::update(WarningState other) {
 	}
 }
 
-std::pair<WarningState, std::optional<uint8_t>> getInitialWarningState(std::string &flag) {
+std::pair<WarningState, std::optional<uint32_t>> getInitialWarningState(std::string &flag) {
 	// Check for prefixes that affect what the flag does
 	WarningState state;
 	if (flag.starts_with("error=")) {
@@ -51,28 +51,22 @@ std::pair<WarningState, std::optional<uint8_t>> getInitialWarningState(std::stri
 	// Is the rest of the string a decimal number?
 	// We want to avoid `strtoul`'s whitespace and sign, so we parse manually
 	char const *ptr = flag.c_str() + equals + 1;
-	uint8_t param = 0;
-	bool warned = false;
+	uint32_t param = 0;
+	bool overflowed = false;
 
-	// The `if`'s condition above ensures that this will run at least once
-	do {
-		// If we don't have a digit, bail
-		if (*ptr < '0' || *ptr > '9') {
-			break;
-		}
-		// Avoid overflowing!
-		if (param > UINT8_MAX - (*ptr - '0')) {
-			if (!warned) {
-				warnx("Invalid warning flag \"%s\": capping parameter at 255", flag.c_str());
-			}
-			warned = true; // Only warn once, cap always
-			param = 255;
+	for (; *ptr >= '0' && *ptr <= '9'; ++ptr) {
+		if (overflowed) {
 			continue;
 		}
-		param = param * 10 + (*ptr - '0');
 
-		++ptr;
-	} while (*ptr);
+		uint32_t c = *ptr - '0';
+		if (param > (UINT32_MAX - c) / 10) {
+			overflowed = true;
+			param = UINT32_MAX;
+			continue;
+		}
+		param = param * 10 + c;
+	}
 
 	// If we reached the end of the string, truncate it at the '='
 	if (*ptr == '\0') {
diff --git a/test/asm/diagnostic-parameter-cap.asm b/test/asm/diagnostic-parameter-cap.asm
new file mode 100644
index 00000000..f3c824e0
--- /dev/null
+++ b/test/asm/diagnostic-parameter-cap.asm
@@ -0,0 +1,4 @@
+opt Wtruncation=256
+section "test", rom0
+db 999
+db -254
diff --git a/test/asm/diagnostic-parameter-cap.err b/test/asm/diagnostic-parameter-cap.err
new file mode 100644
index 00000000..e9c4fbad
--- /dev/null
+++ b/test/asm/diagnostic-parameter-cap.err
@@ -0,0 +1,5 @@
+warning: Invalid warning flag parameter "truncation=256"; capping at maximum 2
+warning: diagnostic-parameter-cap.asm(3): [-Wtruncation]
+    Expression must be 8-bit; use LOW() to force 8-bit
+warning: diagnostic-parameter-cap.asm(4): [-Wtruncation]
+    Expression must be 8-bit; use LOW() to force 8-bit
diff --git a/test/asm/invalid-param.err b/test/asm/invalid-param.err
index f2c6b42f..e92a054f 100644
--- a/test/asm/invalid-param.err
+++ b/test/asm/invalid-param.err
@@ -1,4 +1,4 @@
-warning: Invalid parameter 99 for warning flag "truncation"; capping at maximum 2
+warning: Invalid warning flag parameter "truncation=99"; capping at maximum 2
 warning: invalid-param.asm(2): [-Wtruncation]
     Expression must be 8-bit; use LOW() to force 8-bit
 warning: invalid-param.asm(3): [-Wtruncation]