diff --git a/contrib/seg/expected/seg.out b/contrib/seg/expected/seg.out
index e617dd7e29..2320464dd4 100644
--- a/contrib/seg/expected/seg.out
+++ b/contrib/seg/expected/seg.out
@@ -256,6 +256,13 @@ SELECT '12.34567890123456'::seg AS seg;
  12.3457
 (1 row)
 
+-- Same, with a very long input
+SELECT '12.3456789012345600000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000'::seg AS seg;
+   seg   
+---------
+ 12.3457
+(1 row)
+
 -- Numbers with certainty indicators
 SELECT '~6.5'::seg AS seg;
  seg  
diff --git a/contrib/seg/seg.c b/contrib/seg/seg.c
index 4a8e2be329..91b8a79600 100644
--- a/contrib/seg/seg.c
+++ b/contrib/seg/seg.c
@@ -927,9 +927,13 @@ restore(char *result, float val, int n)
 
 	/*
 	 * Put a cap on the number of significant digits to avoid garbage in the
-	 * output and ensure we don't overrun the result buffer.
+	 * output and ensure we don't overrun the result buffer.  (n should not be
+	 * negative, but check to protect ourselves against corrupted data.)
 	 */
-	n = Min(n, FLT_DIG);
+	if (n <= 0)
+		n = FLT_DIG;
+	else
+		n = Min(n, FLT_DIG);
 
 	/* remember the sign */
 	sign = (val < 0 ? 1 : 0);
diff --git a/contrib/seg/segparse.y b/contrib/seg/segparse.y
index 040cab3904..3115b12ebd 100644
--- a/contrib/seg/segparse.y
+++ b/contrib/seg/segparse.y
@@ -3,6 +3,7 @@
 
 #include "postgres.h"
 
+#include <float.h>
 #include <math.h>
 
 #include "fmgr.h"
@@ -23,6 +24,8 @@
 
 static float seg_atof(const char *value);
 
+static int sig_digits(const char *value);
+
 static char strbuf[25] = {
 	'0', '0', '0', '0', '0',
 	'0', '0', '0', '0', '0',
@@ -63,9 +66,9 @@ range: boundary PLUMIN deviation
 		result->lower = $1.val - $3.val;
 		result->upper = $1.val + $3.val;
 		sprintf(strbuf, "%g", result->lower);
-		result->l_sigd = Max(Min(6, significant_digits(strbuf)), Max($1.sigd, $3.sigd));
+		result->l_sigd = Max(sig_digits(strbuf), Max($1.sigd, $3.sigd));
 		sprintf(strbuf, "%g", result->upper);
-		result->u_sigd = Max(Min(6, significant_digits(strbuf)), Max($1.sigd, $3.sigd));
+		result->u_sigd = Max(sig_digits(strbuf), Max($1.sigd, $3.sigd));
 		result->l_ext = '\0';
 		result->u_ext = '\0';
 	}
@@ -122,7 +125,7 @@ boundary: SEGFLOAT
 		float val = seg_atof($1);
 
 		$$.ext = '\0';
-		$$.sigd = significant_digits($1);
+		$$.sigd = sig_digits($1);
 		$$.val = val;
 	}
 	| EXTENSION SEGFLOAT
@@ -131,7 +134,7 @@ boundary: SEGFLOAT
 		float val = seg_atof($2);
 
 		$$.ext = $1[0];
-		$$.sigd = significant_digits($2);
+		$$.sigd = sig_digits($2);
 		$$.val = val;
 	}
 	;
@@ -142,7 +145,7 @@ deviation: SEGFLOAT
 		float val = seg_atof($1);
 
 		$$.ext = '\0';
-		$$.sigd = significant_digits($1);
+		$$.sigd = sig_digits($1);
 		$$.val = val;
 	}
 	;
@@ -159,5 +162,14 @@ seg_atof(const char *value)
 	return DatumGetFloat4(datum);
 }
 
+static int
+sig_digits(const char *value)
+{
+	int			n = significant_digits(value);
+
+	/* Clamp, to ensure value will fit in sigd fields */
+	return Min(n, FLT_DIG);
+}
+
 
 #include "segscan.c"
diff --git a/contrib/seg/sql/seg.sql b/contrib/seg/sql/seg.sql
index 6fe33e90e4..a027d4de97 100644
--- a/contrib/seg/sql/seg.sql
+++ b/contrib/seg/sql/seg.sql
@@ -60,6 +60,9 @@ SELECT '3.400e5'::seg AS seg;
 -- Digits truncated
 SELECT '12.34567890123456'::seg AS seg;
 
+-- Same, with a very long input
+SELECT '12.3456789012345600000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000'::seg AS seg;
+
 -- Numbers with certainty indicators
 SELECT '~6.5'::seg AS seg;
 SELECT '<6.5'::seg AS seg;