Buffer of a Linestring returns a Polygon with hole artifact

[nbvfgh]

ST_Buffer(geometry g1, float radius_of_buffer):
Computes a POLYGON or MULTIPOLYGON that represents all points whose distance from a geometry/geography is less than or equal to a given distance. A negative distance shrinks the geometry rather than expanding it. A negative distance may shrink a polygon completely, in which case POLYGON EMPTY is returned. For points and lines negative distances always return empty results.

Considering following queries:

DROP TABLE IF EXISTS t1;
CREATE TABLE t1 (geom geometry);
INSERT INTO t1 VALUES (ST_GeomFromText('LINESTRING (1.6 0.7, 0.7 4.7, 145.6 137.7, 146.3 137.1, 146.3 137, 146.6 136.7)', 0));

SELECT ST_Covers(ST_Buffer(t1.geom, 15), t1.geom) FROM t1;
-- expected: {t}

The buffer has created an unexpected hole.

[nbvfgh]

bug tracker: libgeos/geos#1217