diff --git a/frontends/ast/genrtlil.cc b/frontends/ast/genrtlil.cc
index d3982b92b..8ccf74db8 100644
--- a/frontends/ast/genrtlil.cc
+++ b/frontends/ast/genrtlil.cc
@@ -771,8 +771,6 @@ struct AST_INTERNAL::ProcessGenerator
 					if (node->type == AST_CONSTANT && node->is_string) {
 						arg.type = VerilogFmtArg::STRING;
 						arg.str = node->bitsAsConst().decode_string();
-						// and in case this will be used as an argument...
-						arg.sig = node->bitsAsConst();
 						arg.signed_ = false;
 					} else if (node->type == AST_IDENTIFIER && node->str == "$time") {
 						arg.type = VerilogFmtArg::TIME;
diff --git a/kernel/fmt.cc b/kernel/fmt.cc
index 6e9f2924a..8c9b9a510 100644
--- a/kernel/fmt.cc
+++ b/kernel/fmt.cc
@@ -378,7 +378,7 @@ void Fmt::parse_verilog(const std::vector<VerilogFmtArg> &args, bool sformat_lik
 			}
 
 			case VerilogFmtArg::STRING: {
-				if (arg == args.begin() || !sformat_like) {
+				if ((arg == args.begin() || !sformat_like) && !arg->str.empty()) {
 					const auto fmtarg = arg;
 					const std::string &fmt = fmtarg->str;
 					FmtPart part = {};
@@ -407,8 +407,19 @@ void Fmt::parse_verilog(const std::vector<VerilogFmtArg> &args, bool sformat_lik
 							if (++arg == args.end()) {
 								log_file_error(fmtarg->filename, fmtarg->first_line, "System task `%s' called with fewer arguments than the format specifiers in argument %zu require.\n", task_name.c_str(), fmtarg - args.begin() + 1);
 							}
-							part.sig = arg->sig;
-							part.signed_ = arg->signed_;
+							switch (arg->type) {
+							case VerilogFmtArg::STRING:
+								part.sig = arg->str.empty() ? arg->sig : Const(arg->str);
+								part.signed_ = false;
+								break;
+							case VerilogFmtArg::INTEGER:
+								part.sig = arg->sig;
+								part.signed_ = arg->signed_;
+								break;
+							default:
+								// requires `%t`/`%T` which is enforced later on
+								break;
+							}
 
 							for (; i < fmt.size(); i++) {
 								if (fmt[i] == '-') {
@@ -472,9 +483,10 @@ void Fmt::parse_verilog(const std::vector<VerilogFmtArg> &args, bool sformat_lik
 								}
 								break;
 							}
-							if (i == fmt.size()) {
+							if (i == fmt.size())
 								log_file_error(fmtarg->filename, fmtarg->first_line, "System task `%s' called with incomplete format specifier in argument %zu.\n", task_name.c_str(), fmtarg - args.begin() + 1);
-							}
+							if (arg->type == VerilogFmtArg::TIME && part.type != FmtPart::VLOG_TIME)
+								log_file_error(fmtarg->filename, fmtarg->first_line, "System task `%s' called with format character `%c' in argument %zu, but the argument is $time or $realtime.\n", task_name.c_str(), fmt[i], fmtarg - args.begin() + 1);
 
 							if (part.padding == '\0') {
 								if (has_leading_zero && part.justify == FmtPart::RIGHT) {
diff --git a/kernel/fmt.h b/kernel/fmt.h
index 2d4b24979..e2b0025fe 100644
--- a/kernel/fmt.h
+++ b/kernel/fmt.h
@@ -26,6 +26,17 @@ YOSYS_NAMESPACE_BEGIN
 
 // Verilog format argument, such as the arguments in:
 //   $display("foo %d bar %01x", 4'b0, $signed(2'b11))
+//
+// Argument mapping:
+//
+//  (1) quoted string literal                   -> STRING, literal in `str`
+//  (2) type `string` or unpacked `byte` array  -> STRING, value in `sig` (SystemVerilog extension)
+//  (3) `$time` `$realtime`                     -> TIME, `realtime` selects between the two
+//  (4) any other expression                    -> INTEGER, value in `sig`
+//
+// We need to distinguish (1) and (2) because that influences interpretation
+// of escape characters (only interpreted in (1)).
+//
 struct VerilogFmtArg {
 	enum {
 		STRING  = 0,
@@ -40,8 +51,10 @@ struct VerilogFmtArg {
 	// STRING type
 	std::string str;
 
-	// INTEGER type
+	// INTEGER/STRING type
 	RTLIL::SigSpec sig;
+
+	// INTEGER
 	bool signed_ = false;
 
 	// TIME type