diff --git a/src/threading/formatters/Ascii.cc b/src/threading/formatters/Ascii.cc
index b9075f97da..61cd7846fa 100644
--- a/src/threading/formatters/Ascii.cc
+++ b/src/threading/formatters/Ascii.cc
@@ -261,7 +261,7 @@ Value* Ascii::ParseValue(const string& s, const string& name, TypeTag type, Type
 
 	case TYPE_COUNT:
 		val->val.uint_val = strtoull(start, &end, 10);
-		if ( CheckNumberError(start, end) )
+		if ( CheckNumberError(start, end, true) )
 			goto parse_error;
 		break;
 
@@ -292,7 +292,7 @@ Value* Ascii::ParseValue(const string& s, const string& name, TypeTag type, Type
 			start = numberpart.c_str();
 			}
 		val->val.port_val.port = strtoull(start, &end, 10);
-		if ( CheckNumberError(start, end) )
+		if ( CheckNumberError(start, end, true) )
 			goto parse_error;
 		}
 		break;
@@ -473,7 +473,7 @@ parse_error:
 	return nullptr;
 	}
 
-bool Ascii::CheckNumberError(const char* start, const char* end) const
+bool Ascii::CheckNumberError(const char* start, const char* end, bool nonneg_only) const
 	{
 	MsgThread* thread = GetThread();
 
@@ -491,6 +491,18 @@ bool Ascii::CheckNumberError(const char* start, const char* end) const
 	if ( (*end != '\0') )
 		thread->Warning(thread->Fmt("Number '%s' contained non-numeric trailing characters. Ignored trailing characters '%s'", start, end));
 
+	if ( nonneg_only ) {
+		// String may legitimately start with whitespace, so
+		// we skip this before checking for a minus sign.
+		const char* s = start;
+		while ( s < end && isspace(*s) )
+			s++;
+		if ( *s == '-' ) {
+			thread->Warning(thread->Fmt("Number '%s' cannot be negative", start));
+			return true;
+		}
+	}
+
 	if ( errno == EINVAL )
 		{
 		thread->Warning(thread->Fmt("String '%s' could not be converted to a number", start));
diff --git a/src/threading/formatters/Ascii.h b/src/threading/formatters/Ascii.h
index c2d6218af2..4cfe2cec04 100644
--- a/src/threading/formatters/Ascii.h
+++ b/src/threading/formatters/Ascii.h
@@ -54,7 +54,7 @@ public:
 	                          TypeTag type, TypeTag subtype = TYPE_ERROR) const;
 
 private:
-	bool CheckNumberError(const char* start, const char* end) const;
+	bool CheckNumberError(const char* start, const char* end, bool nonneg_only = false) const;
 
 	SeparatorInfo separators;
 };
diff --git a/testing/btest/Baseline/scripts.base.frameworks.input.invalidnumbers/.stderrwithoutfirstline b/testing/btest/Baseline/scripts.base.frameworks.input.invalidnumbers/.stderrwithoutfirstline
index ca44cfc3a2..0bac11e721 100644
--- a/testing/btest/Baseline/scripts.base.frameworks.input.invalidnumbers/.stderrwithoutfirstline
+++ b/testing/btest/Baseline/scripts.base.frameworks.input.invalidnumbers/.stderrwithoutfirstline
@@ -5,5 +5,7 @@ warning: ..<...>/Input::READER_ASCII: Number '9223372036854775801TEXTHERE' conta
 warning: ..<...>/Input::READER_ASCII: Number '1Justtext' contained non-numeric trailing characters. Ignored trailing characters 'Justtext'
 warning: ..<...>/Input::READER_ASCII: String 'Justtext' contained no parseable number
 warning: ..<...>/Input::READER_ASCII: Could not convert line 'Justtext	1' of ../input.log to Val. Ignoring line.
+warning: ..<...>/Input::READER_ASCII: Number ' -18446744073709551612' cannot be negative
+warning: ..<...>/Input::READER_ASCII: Could not convert line '9223372036854775800	 -18446744073709551612' of ../input.log to Val. Ignoring line.
 received termination signal
 >>>
diff --git a/testing/btest/Baseline/scripts.base.frameworks.input.invalidnumbers/out b/testing/btest/Baseline/scripts.base.frameworks.input.invalidnumbers/out
index 366f93130c..3bc16fee9d 100644
--- a/testing/btest/Baseline/scripts.base.frameworks.input.invalidnumbers/out
+++ b/testing/btest/Baseline/scripts.base.frameworks.input.invalidnumbers/out
@@ -1,5 +1,4 @@
 ### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
 {
-[9223372036854775801] = [c=1],
-[9223372036854775800] = [c=4]
+[9223372036854775801] = [c=1]
 }
diff --git a/testing/btest/Baseline/scripts.base.frameworks.input.port-embedded/zeek..stderr b/testing/btest/Baseline/scripts.base.frameworks.input.port-embedded/zeek..stderr
index 2695e2a3d1..b32babb871 100644
--- a/testing/btest/Baseline/scripts.base.frameworks.input.port-embedded/zeek..stderr
+++ b/testing/btest/Baseline/scripts.base.frameworks.input.port-embedded/zeek..stderr
@@ -1,3 +1,5 @@
 ### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
 warning: ../input.log/Input::READER_ASCII: Port '50/trash' contained unknown protocol 'trash'
+warning: ../input.log/Input::READER_ASCII: Number '-1' cannot be negative
+warning: ../input.log/Input::READER_ASCII: Could not convert line '1.2.3.8	-1/tcp' of ../input.log to Val. Ignoring line.
 received termination signal
diff --git a/testing/btest/scripts/base/frameworks/input/invalidnumbers.zeek b/testing/btest/scripts/base/frameworks/input/invalidnumbers.zeek
index 16a3cda1de..822e99f511 100644
--- a/testing/btest/scripts/base/frameworks/input/invalidnumbers.zeek
+++ b/testing/btest/scripts/base/frameworks/input/invalidnumbers.zeek
@@ -4,6 +4,8 @@
 # @TEST-EXEC: sed 1d .stderr > .stderrwithoutfirstline
 # @TEST-EXEC: TEST_DIFF_CANONIFIER=$SCRIPTS/diff-remove-abspath btest-diff .stderrwithoutfirstline
 
+# Note the tab+space separator in the last line of the following is
+# intentional. It verifies our whitespace handling.
 @TEST-START-FILE input.log
 #separator \x09
 #fields	i	c
@@ -11,7 +13,7 @@
 12129223372036854775800	121218446744073709551612
 9223372036854775801TEXTHERE	1Justtext
 Justtext	1
-9223372036854775800	-18446744073709551612
+9223372036854775800	 -18446744073709551612
 @TEST-END-FILE
 
 redef exit_only_after_terminate = T;
diff --git a/testing/btest/scripts/base/frameworks/input/port-embedded.zeek b/testing/btest/scripts/base/frameworks/input/port-embedded.zeek
index ef4b0a0651..5e8ad05fb7 100644
--- a/testing/btest/scripts/base/frameworks/input/port-embedded.zeek
+++ b/testing/btest/scripts/base/frameworks/input/port-embedded.zeek
@@ -9,6 +9,7 @@
 1.2.3.5	52/udp
 1.2.3.6	30/unknown
 1.2.3.7	50/trash
+1.2.3.8	-1/tcp
 @TEST-END-FILE
 
 redef exit_only_after_terminate = T;